When you're writing a list, use commas between each item in your list. Use and between the last two items, instead of a comma.
Today I need to buy milk, bread, apples and potatoes.
In longer sentences, you can use commas to separate out extra information to make the sentence easier to read.
Jack the Ripper, the most famous killer in history, was never caught.
A clause is the building block for a sentence. Commas can be used to break up sentences that have more than one clause, to make them easier to read.
When Albert saw the food, his tummy started to rumble.
This sentence has a subordinate clause: When Albert saw the food. Subordinate clauses do not make sense on their own because they need the main part of the sentence (his tummy started to rumble) to be completely clear. When you have two clauses like these, they need to be separated by a comma.
If the clauses make sense on their own, you donít need to use a comma. Write the clauses as separate sentences instead.
Albert was excited about eating. He wanted to use a knife and fork.