There are some general rules which you can apply when using the comma. However, you will find that in English there are many other ways to use the comma to add to the meaning of a sentence or to emphasise an item, point, or meaning.
Although we are often taught that commas are used to help us add 'breathing spaces' to sentences they are, in fact, more accurately used to organise blocks of thought or logical groupings. Most people use commas to ensure that meaning is clear and, despite grammatical rules, will drop a comma if their meaning is retained without it.
Separating main clauses
Main clauses that are joined together with and or but do not normally have a comma before the conjunction unless the two clauses have different subjects.
- You go out of the door and turn immediately left.
- It was cold outside, but we decided to go out for a walk anyway.
Separating subordinate clauses from main clauses
Commas are normally used if the subordinate clause comes before the main clause.
- If you have any problems, just call me.
- Just call me if you have any problems.
Sometimes a comma is used even when the main clause comes first, if the clauses are particularly long.
- We should be able to finish the work by the end of the week,
if nothing unexpected turns up between now and then.
Separating relative clauses from main clauses
Commas are used to mark off non-defining relative clauses (see Relative clauses). This is the type of clause that adds to information about a noun or noun phrase.
- My next-door neighbour, who works from home, is keeping an eye on the house while we’re away.
- She moved to Los Angeles, where she was immediately signed as a singer songwriter.
Commas are not required in defining relative clauses (see Relative clauses), since these simply postmodify the noun.
- Let’s make sure the money goes to the people who need it most.
- The computer (that) I borrowed kept on crashing.
Separating items in a list
Commas are used to separate three or more items in a list or series.
- She got out bread, butter, and jam (but bread and butter).
Note that the comma is often not given before the final and or or.
- They breed dogs, cats, rabbits and hamsters.
- We did canoeing, climbing and archery.
Separating adjectives
Commas are used between adjectives, whether they come before the noun (i.e. used attributively) or after a linking verb (i.e. used predicatively).
- It was a hot, dry and dusty road.
- It’s wet, cold and windy outside.
A comma is not usually used before an adjective that is followed by and.
With adverbials
When an adverbial such as however, therefore or unfortunately modifies a whole sentence, it is separated from the rest of the sentence by a comma.
- However, police would not confirm this rumour.
- Therefore, I try to avoid using the car as much as possible.
With question tags and short responses
Commas are used before question tags and after yes or no in short responses.
- It’s quite cold today, isn’t it?
- He’s up to date with all his injections, isn’t he?
- Are you the mother of these children? – Yes, I am.
- You’re Amy Osborne, aren’t you? – No, I’m not.
With vocatives
Commas are used to separate the name of a person or group being addressed from the rest of the sentence.
- And now, ladies and gentlemen, please raise your glasses in a toast to the happy couple.
- Come on, Olivia, be reasonable.
- Dad, can you come and help me, please?
With discourse markers
Commas are used to separate discourse markers like Well and Now then from the rest of the sentence.
- Well, believe it or not, I actually passed!
- Now then, let’s see what’s on TV tonight.
- Actually, I quite enjoyed it.
In reported speech
Commas are used to follow direct speech (if there is no question or exclamation mark after the quotation), or to show that it comes next.
- ‘I don’t understand this question,’ said Peter.
- Peter said, ‘I don’t understand this question.’
- ‘You’re crazy!’ Claire exclaimed.
- ‘What do you think you’re doing?’ Dad bellowed.
It is also possible to punctuate reported speech of the type Peter said, ‘…’ using a colon instead of a comma. This is a particularly common practice in American English.
- Peter said: ‘Dream on.’
In dates
A comma must be used between the day of the month and the year, when the two numbers are next to each other.
- March 31, 2011