This is a very common punctuation mark which sometimes gives problems.
1. The main use of the comma is to separate items in a list:
I have seen Cats, Phantom of the Opera, Starlight Express and Evita.
You can also use commas to separate actions:
When I go on holiday I like to lie on the beach, read a book, go for a swim, take long walks and try a different restaurant every night. What about you?
2. One use of the comma which causes learners a lot of problems is its use before the word ‘which’. In some languages you always use a comma in this situation, but in English you may, or may not, depending on the situation, and the difference may be important.
2.1 Madonna’s concert, which is on Monday, is sold out.
This is called a non-defining relative clause. The phrase ‘which is on Monday' gives you extra information but doesn’t tell you which concert it is. We can assume that there is only one concert. In this case the commas act just like brackets: Madonna’s concert (which is on Monday) is sold out.
2.2. Madonna’s concert which is on Monday is sold out
Madonna’s concert which is on Friday is not sold out
This is called a defining relative clause. There are two concerts and the phrase starting with ‘which’ gives you essential information to help you see which is which.
3. Don’t use a comma to separate sentences. This is a modern trend (unfortunately) but wrong: the comma suggests only quite a light pause.
E.g. The photocopier we bought last year has stopped working, the engineers are coming tomorrow.
This is much better as two separate sentences:
The photocopier we bought last year has stopped working. The engineers are coming tomorrow.
4. Remember that in the UK when we write a number in figures we never use a comma to separate whole numbers from decimal fractions:
E.g. Don’t write 47,2. Write: 47.2 (forty-seven point two).
We often use a comma to separate the hundreds from the thousands in a 4-figure number (and to break up long numbers):
2,546 (Two thousand, five hundred and forty-six).
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