Let's consider the following sentence: "I sat on the grass in the park and read a book." Should there be a comma before 'and' or not? I found this sentence in a British book but I was told by a native English speaker from the States that this is a mistake. Is this simply a question of difference between BrE and AmE, or is there something more to it? Oh, and an additional question: would providing the subject in the second part of the sentence make a difference? Say, if the sentence were: "I sat on the grass in the park and I read a book", would it require a comma before 'and'?