text: Actually the sentence you have here is not correct, it should be "Jane and Paul were late because they forgot the present" or "Jane and Paul were late; they forgot the present".  The semi-colon SUBSTITUTES for a coordinating conjunction at the sentence level:  "I thought about leaving the party; the hostess was quite boring", versus "I thought about leaving the party because the hostess was quite boring."  And, actually, neither "for" nor "because" are coordinating conjunctions anyway.  However, there is one instance where you would use a semicolon followed by a coordinating conjunction--in complex list.  "John ate oysters, mussels, and scallops; a bean stew; a tomato, celery, and lettuce; and a good steak."  Note that the semicolon precedes a coordinating conjunction.