when the second one explains or summarizes the first
ex: This is my point: if we buy judy a plane, she might take us for a ride.
to join two related complete thoughts
ex: Toby felt very tired; he decided to play soccer with his friends anyway
You put a semi-colon before and a comma after
ex: Mark likes bananas; however, he's not in the mood for one now.
place a comma before and after
ex: Dave loves his dog. Brining it to class, however, was a mistake
there is no comma
ex: My cat that has green eyes purrs when I rub its belly.
befor and after the clause
ex: The Jewels, which cost a lot of money, were stolen from the safe.
a comma
Semi colon or a period