Punctuation can cause a lot of problems. The more complex the sentence, the more scope there is for problems to arise. So, if you do not feel confident about punctuation the best advice is:

• write short sentences, and use only full stops, commas and quotation marks;
• if you run into problems with a complex or lengthy sentence, then rearrange the sentence to avoid the problems.

Those points having been made, the correct use of colons, semicolons, commas and dashes can add variety to your writing. You really should know how to use these things. They are explained below.

Commas: when to use them, when not to use them

Commas are used a a 'soft break' within a sentence, rather like a breathing space when speaking. However, this legitimate (but almost always unnecessary) use of the comma leads to two common errors.

Error 1. When a comma separates a verb from its subject or object

Example (from an Honours student’s essay) ‘He noticed that tobacco plants infected with a mild strain of tobacco mosaic virus, did not exhibit severe disease symptoms when challenged with a severe strain.’

Explanation The simplest possible type of sentence is Dogs bark. In this sentence Dogs (a noun) is the subject of the verb bark. You would never write this sentence as Dogs, bark. You know this intuitively, but this is what the student did in the essay. The grammatical error was to separate the subject of a verb (in this case tobacco plants infected with a mild strain of tobacco mosaic virus) from the verb (did not exhibit) by a comma.

The same applies to verbs and their objects. You would never write the sentence 'I like, dogs.'

Solution When you check through your essay, look at each sentence with 'Dogs, bark' and 'I like, dogs' in mind. Then you will spot your errors easily.

Error 2. Use of a comma in place of a full-stop

We have dealt with this before, but it is so common that it bears repeating.

Example (from a 3rd Year essay) ‘The fungus found on these trees was shown to be non-virulent, if this fungus was transferred to a tree infected with virulent fungi the infection was arrested.’

Explanation There are two separate sentences, and two separate points here. They have been joined by a comma. You might argue that this is not serious, because we still understand the writer's meaning. But it would not impress anyone - it breaks the most elementary rule of writing, that sentences deal with a single train of thought and end with full stops, not commas.

Solution Make one point, use a full-stop, then make the next point. Alternatively, you can use a semicolon in place of a full-stop; this would indicate that the two points are sufficiently related to deserve being braced in a single sentence. [Notice how I did this in the last sentence, and again here; semicolons are underused but can add variety to your writing, and avoid staccato.]

Legitimate uses of commas

1. Use commas in lists

Example: At the zoo we saw lions, tigers, bears and elephants. [Note: usually we do not put a comma before 'and' but if we did so (bears, and elephants) the effect would be to make some point about seeing elephants. It could be to emphasise that we even saw elephants! Or it could be to suggest that of course we saw elephants.]

2. Use commas like brackets

Example: Mitochondria, which generate ATP, are easily seen after staining with the dye Janus green. [Note: in this case we use commas to isolate the extra information that is not essential to the meaning of the sentence. The commas and everything within them could be removed, and still the sentence would make sense both scientifically and grammatically: Mitochondria are easily seen after staining with the dye Janus green. Note, also, that the use of a pair of commas, serving as brackets, does not break the golden rule that a verb should not be separated from its subject or object by a comma. I did this twice in the last sentence, and you will see that the sentence still makes perfect sense when the commas and everything between them are removed: Note that the use of a pair of commas does not break the golden rule that a verb should not be separated from its subject or object by a comma.]

3. Use commas after an introductory word or phrase, or sometimes before a final statement

Example: Read the heading again. I used a comma between 'phrase' and 'or', to add additional information. [I've done it again!] Note that, in this case, the comma helps to avoid momentary confusion. If I had written 'word or phrase or sometimes...' without the comma you might have thought (for a moment) that I was giving a choice of three equivalent things (word or phrase or sometimes).

Another example: Having water-repellent fur, polar bears are well adapted to maintaining their body heat when swimming in Arctic waters. [Note: the sentence is fine, but what I did was dangerous. I used what is called a hanging (or unattached) participle - an initial phrase (Having water-repellent fur) to give some information about polar bears. Had I been less careful, I might have written: Having water-repellent fur, we can assume that polar bears are well adapted... Now the implication is that we have water-repellent fur.

Solution: Beware of hanging participles - they will hang onto the first thing they meet, and that can make us look silly!]

4. Use commas to isolate words such as 'however', 'therefore', 'for example'. IMPORTANT: lots of people get this wrong!

This is really just another case of using commas like brackets (point 2 above) to isolate words that might give extra meaning or 'flavour' but that are not essential to the grammatical meaning of a sentence.

Examples Below are examples of incorrect sentences and the corresponding correct sentences.

Wrong. It was shown, however that the bacterium was virulent.
Wrong. It was shown however, that the bacterium was virulent.
Correct. It was shown, however, that the bacterium was virulent.
Correct However, it was shown that the bacterium was virulent.

Wrong. Flies for example, have one pair of wings.
Correct Flies, for example, have one pair of wings.
Correct For example, flies have one pair of wings.

Wrong. We see therefore, that flies conform to the normal pattern for Diptera.
Correct We see, therefore, that flies conform to the normal pattern for Diptera.
Correct Therefore, we see that flies conform to the normal pattern for Diptera.

Explanation The rule is simple. You use terms such as however, therefore, thus, nevertheless, even so, consequently, also, etc. etc. to add extra information and (or) to make your sentences flow. But they are not essential to the (grammatical) meaning of the sentence, so they should be separated from it by commas (only one being needed if these words are used at the start or end of the sentence).

In contrast, words like ‘but’ and ‘and’ are conjunctions (joining words) and so are integral parts of the meaning of a sentence. In each example below you will see that but or and adds meaning to the sentence, and cannot be removed without changing the meaning.

Correct I could do it but I will not do it.
Also correct: I could do it, but I will not do it. [Here the comma is used to emphasise a contrast - I could, but I will not.]
Correct: I can do it, and I will do it.
Also correct: I can do it and I will do it. [But the previous sentence makes the point better]

Wrong: I could do it, but, I will not do it. [Try to remove 'but' (which the writer suggests we can do because it is placed between commas) and the sentence would become two separate sentences strung together: I could do it I will not do it.]
Wrong: I could do it, however, I will not do it. [As above, try to remove 'however' and you get nonsense]
Wrong: I could do it, however I will not do it. [Wrong because 'but' is the conjunction that you should use in the flow of a sentence]
Correct: I could do it; however, I will not do it.

5. Problems in the use of 'for example'. IMPORTANT: lots of people get this wrong!

The following two sentences are correct.

‘Mendel showed this in several ways - for example, by a back-cross.’
‘Mendel showed this in several ways; for example, by a back-cross.’

They are correct because the part that follows the hyphen or the semicolon is valuable extra information but could be removed without making nonsense of the sentence. We would still learn that Mendel showed this in several ways.

The following also makes sense grammatically.

‘Mendel showed this, for example, by a back-cross.’

It makes sense because 'for example' could be removed and we would still learn that Mendel showed this by a back-cross. BUT it is an example of poor writing because the meaning is unclear. Are we being told that Mendel is an example of one of the people who showed it? Or are we being told that Mendel showed it and an example of one of the ways in which he showed it was by a back-cross?

Now go back to the two correct and meaningful sentences:

‘Mendel showed this in several ways - for example, by a back-cross.’
‘Mendel showed this in several ways; for example, by a back-cross.’

The use of the dash or semicolon adds a break to our sentence (See semicolon) and shows us what 'for example' refers to.

To continue on this theme for a little longer, ask yourself what is wrong with the following:

‘Mendel showed this in several ways, for example, by a back-cross.’

It is wrong because it suggests that we could remove 'for example' (it lies between commas) and the sentence would still have its original meaning. In fact, it would then mean that Mendel showed it in several ways, all involving a back-cross. Was that what the writer meant to say?

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Colon and semicolon

In the hierarchy of 'breaks' or 'stops' we go from the full stop (absolute break) down to the colon (seldom used), then the semicolon, and then the comma.

The colon (:)

The colon is useful for two main purposes: (1) to introduce what follows (which is how I have just used it) and (2) to balance or contrast two points.

The following are examples of the second type.

Birds have wings: fish have fins.
Amy has blue eyes: I have brown.
The Chihuahuan desert has summer rainfall: the Mojave desert has winter rains.

You will note that, although these sentences are rather pleasing and dramatic, the colon is not really necessary. In each case we could say the same thing in several different ways.

Birds have wings whereas fish have fins.
Amy has blue eyes and I have brown.
The Chihuahuan desert has summer rainfall but the Mojave desert has winter rains.

Because of this redundancy, the modern trend is to use the colon only for making lists, and even then it can cause problems. See Making lists.

The semicolon (;)

The semicolon has almost disappeared from the landscape of writing; this is unfortunate, because the semicolon can be extremely useful. Its main use is to link two sentences or themes (as I did in the last sentence) that naturally fit together but that otherwise would require two separate sentences.

Ironically, students often link sentences together (which shows that they see the value of this). But they use a comma instead of the semicolon. Here is an example.

Dogs bark when they are frightened, I don't like that. (Wrong)
Dogs bark when they are frightened; I don't like that. (Correct)

Guidance: Don't overdo them, but semicolons will improve your writing by adding variety; they enable you to avoid strings of short sentences.

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The apostrophe

The apostrophe (’) is used in two ways.

• To show where letters have been missed out of a word - for example, hasn't instead of has not, isn't instead of is not.
• To show that something belongs to something else - for example, John's list, the horse's mouth.

That's (that is) all very straightforward, but several problems crop up.

1. Its and it's (IMPORTANT - lots of people get this wrong)

it's means 'it is'
its means 'belonging to it'

There is no easy way to remember this, so I suggest that you avoid the problem altogether: always write it is, when that's what you mean. Then its (belonging to it) will look after itself. No apostrophe is needed; it couldn't be easier!

In short, you will have problems with its and it's ONLY if you are too lazy to write it is!

2. Plurals

(a) When the plural ends with an "s" then we put the apostrophe after the "s". For example, the mouth belonging to a single horse is 'The horse's mouth' but the mouths belonging to several horses are 'The horses' mouths'.

As another example, the teeth belonging to one animal are the animal's teeth, but the teeth belonging to several animals are animals' teeth, and the kennel belonging to two dogs is the dogs' kennel.

(b) If the plural does not end in an "s", then you add an apostrophe and an "s".

For example, the room that men use is the men's room. The children sleep in the children's bedroom (or bedrooms). The people's story is the story of the people.[But sometimes we speak of different types of people - for example, the Soviet people and the people of the South-Sea Islands. Then (and only then) we could speak of the peoples' story (or stories).

3. Personal pronouns

A book belonging to her is hers, a book belonging to them is theirs. Similarly, your book is yours, and our book is ours. No apostrophe is needed in these cases. And - let's repeat it once more - a thing belonging to it is its. Again, no apostrophe.

4. Talking about years

It is wrong to write 'In the 1990's there was much turmoil.' The use of an apostrophe here suggests that something belongs to the 1990s, whereas we are writing about the 1990s as a span of years.
It is correct to write 'In the 1990s...'

5. Other problem cases

A problem sometimes arises when people's names end with an "s". For example, the book that belongs to Mr Jones is Mr Jones's book. The hat that belongs to Mrs Bass is Mrs Bass's hat. But what about the book that belongs to Mr and Mrs Jones? I would call this the Jones's book. But whenever you get into potential problem areas such as this it is wise to play safe by rearranging your sentence - write "the book that belongs to Mr and Mrs Jones."

Another problem arises with words such as "species". The habitat of a species can be written as the species' habitat (because to write species's habitat would be ridiculous - just try saying it!).

In truth, you can avoid all these potential problems - and save yourself a lot of time and angst - just by rearranging your sentence. For example, do you really need to write the species' habitat, when the habitat of the species will avoid all difficulties?

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Making lists

Lists cause lots of problems, but only because people abandon the normal rules of punctuation. Stick to those rules and you will never go wrong. Here are several correct ways to list things.

Example 1 (strongly recommended)

Birds are interesting for the following reasons.

1. They lay eggs.
2. They have feathers.
3. They do not have fur.

Note: often this is the best way of making a list, because you might want to write two or three sentences on each item - or, at least, on some of them. You can do this because you finished the first sentence (Birds are interesting for the following reasons.) with a full stop.

Example 2

Birds are interesting because: (a) they lay eggs, (b) they have feathers, (c) they do not have fur.

Note: in this case we have one sentence. It starts with a Capital letter, it ends with a full stop, and we have not broken any rule of punctuation.

Example 3

Birds are interesting because:
(a) they lay eggs,
(b) they have feathers,
(c) they do not have fur.

Note: this is similar to Example 2, but could be used for a long list of things.

Example 4

Birds are interesting because:

• they lay eggs, which are hard-shelled, in contrast to those of reptiles;
• they have feathers, which serve both for flight and for thermoregulation;
• they do not have fur, in contrast to mammals.

Note: this list shows how, again, we can produce a single sentence without breaking the rules of punctuation. By using semicolons at the end of the first two points we could use commas within the listed points. [We could have used (a), (b) etc. instead of the bullets.]

One final point about lists:

Lists are always neater if the listed things are equivalent. For example, the following is not a tidy list.

Birds are interesting because:
(1) they lay eggs,
(2) the wings are feathered,
(3) birds' eggs are oval.

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Dashes and brackets

Dashes and brackets serve the same function - to add extra information that is not essential to the meaning of a sentence. You always use brackets in pairs, but you can use a single dash (as I did in the last sentence) if the extra (non-essential) information runs to the end of the sentence. Some examples follow.

1. Several ions (e.g. chloride, nitrate, sulphate) can affect the sensitivity of the assay.
2. The ions mentioned earlier - i.e. chloride, nitrate, sulphate - can affect the sensitivity of the assay.

Note that in both sentences we could have used either brackets or dashes. Note also that we can use abbreviations (i.e., e.g.) within brackets or dashes, whereas it would be difficult to use these abbreviations in the normal flow of a sentence. And, finally, note that e.g. and i.e. mean entirely different things!

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