Commas

The four jobs that cover most cases.

Where they go

Commas separate items in a list, join two sentences alongside a conjunction, set off an introductory phrase, and fence an aside that could be lifted out.

The comma before and in a list is the serial comma, optional but often clearer.
If the aside can be removed without changing the meaning, it takes commas on both sides.

Where they do not

Do not put a comma between a subject and its verb, and do not join two full sentences with a comma alone. When in doubt, read it aloud and see whether you pause.