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.
Separate phrases, words, or clauses in lists
When making a list, commas are the most common way to separate one list item from the next. The final two items in the list are usually separated by "and" or "or", which should be preceeded by a comma. Amongst editors this final comma in a list is known as the "Oxford Comma".
A series of independent clauses (sentences)
- I met Harry, we went for a swim together, and afterwards Harry went home.
- I like your son, I might even love him, but he is not a very good soccer player.
a series of nouns
- For dinner I had soup, fish, chicken, dessert, and coffee.
- This afternoon I went to Oxford Circus, Picadilly, Hamstead, and Gatwick Airport.
a series of adjectives
A list of adjectives usually requires commas. However, if an adjective is modifying another adjective you do not separate them with a comma (sentence 3).
- She was young, beautiful, kind, and intelligent.
- The house we visited was dark, dreary, and run-down.
- She was wearing a bright red shirt.
a series of verbs
- Tony ran towards me, fell, yelled, and fainted.
- The boy leapt, spun, twisted, and dove into the water.
a series of phrases
- The car smashed into the wall, flipped onto its roof, slid along the road, and finally stopped against a tree.
- The dog leapt into the air, snatched the frisbee in its mouth, landed, and ran off into the forest.
Use a comma to enclose non-defining relative clauses and other non-essential details and comments. The comma is placed on either side of the insertion.
- China, one of the most powerful nations on Earth, has a huge population.
- Jason's grandmother, who was born in 1930, lived through the Second World War.
- Cats, unlike dogs, do not respect their masters.
- My friend, Jim, likes to go scuba diving.
- Hearing that her father was in hospital, Jane left work immediately.
- Walking to the bus stop that morning, Sam knew it was going to be a special day.
- She lives in Paris, doesn't she?
- We haven't met, have we?
- Yes, I will stay a little longer, thank you.
- No, he isn't like other boys.
- Wait, I didn't mean to scare you.
A final warning
Putting a comma in the wrong place can lead to a sentence with a completely different meaning, look at these two sentences:
I detest liars like you; I believe that honesty is the best policy. = I detest you because you are a liar.
I detest liars, like you; I believe that honesty is the best policy. = You and I both detest liars.