Commas, semi-colons and colons are the sentence tidiers. Used correctly, they'll give your written language the 'punctuation' that pauses, voice modulations and gestures provide when you speak.

Commas

There are two simple rules to remember.

1. Commas are used to separate the items in lists.

The last two items are usually separated by an 'and' or 'or'. Often, you need a comma before the 'and', either because the last or second to last item is a double one, and itself requires an 'and', or because the comma helps to prevent ambiguity.

There were several things missing from the table: mayonnaise, mustard, butter, and salt and pepper.

The things he hates most are warm beer, hot curries, cold dinners, and women.

Without the comma, the last two items might be understood as cold dinners and cold women. (Strictly speaking, if "cold" were meant to apply to the women also, the sentence should read "... hot curries, and cold dinners and women", but the comma helps to reinforce the distinction.)

If the sentence won't be ambiguous without it, then the conjunction provides adequate pause, and you can drop the comma.

Please bring pencil, paper, ruler and rubber.

The rule applies whether the list consists of words, phrases or clauses:

I arrived home, fed the cat, ate my dinner, turned out the lights and went to bed.

2. Commas are used either side of a parenthetic expression.

A parenthetic expression is one that is not essential to the meaning of the sentence.

She gave me a refund, which wasn't what I wanted at all.

Men, who are liars, should not be trusted.

If you leave out the commas, you're signalling to your readers that the expression is essential to your intended meaning. In the second example above, the sentence says that men should not be trusted, and explains why. If we remove the commas, it says something different: that a certain class of men - those who are liars - should not be trusted:

Men who are liars should not be trusted.

If you address somebody in a sentence, that is also a parenthesis and follows the same rule.

Today, Susan, we are going to finish this report.

Words and phrases such as 'moreover' and 'on the other hand' are always parenthetic, so always put commas around them.

Semi-colons

A semi-colon is used when you've introduced an idea that expands on the previous one or is closely related to it, and a full-stop would create too strong a break. For example:

He said it wouldn't be a good idea; in fact, it would actually be damaging.

Use a comma, not a semi-colon, if you've put an 'and' or other conjunction between the two clauses.

Semi-colons can also be used to group similar items in a list:

Course participants should bring paper and pencils; knife, fork and spoon; plate and mug; and any personal items they will need for the day.

Colons

Colons are used before long or formal quotations, and when introducing lists or specifying something that has just been mentioned:

Herbert Spencer said: "How often misused words generate misleading thoughts."

The following items are available from the school shop: pies, sandwiches, cakes, muesli bars, yoghurt and fruit.

It was just what he wanted: a swiss army knife.