When two complete sentences are joined together using a co-ordinating conjunction (for, and, nor, but, or, yet, so), it should have a comma before it.
- I have spoken to Sarah, and she has confirmed the dates.
(I have spoken to Sarah. + She has confirmed the dates. Two simple sentences joined together with and.)
Compare:
- I wanted to go to the beach, but I couldn’t afford to pay for a hotel room.
(There should be a comma before but because I couldn’t afford to pay for a hotel room is a complete sentence.) - I wanted to go to the beach but couldn’t afford to pay for a hotel room.
(There shouldn’t be a comma before but because couldn’t afford to pay for the hotel room is not a complete sentence.)
