The conjunction and is one of the FANBOYS. We use and to join two simple sentences into a compound sentence.
A compound sentence has two complete thoughts.
Bob is writing a story, and Mary is drawing a picture.
Do you see how this sentence has two things happening? The conjunction and joins the two short sentences together. Whenever we use the conjunction and to make a compound sentence, we must use a comma with and.
I am eating ice cream, and Mom is drinking coffee.
We have a second subject doing something after and – Mom drinking coffee – so we need a comma with and.
Do not use a comma when and joins two subjects together.
Bob and Sam are playing music with their guitars.
In this sentence, there is no subject doing something after and – so we don’t use a comma.
Jack went downtown and bought a new coat.
Only use ,and when joining two simple sentences into a compound sentence.