Typical problem: Deciding whether to put a comma before the conjunction in a compound sentence

1. The windows were open so the rain came in the house.

2. The windows were open, so the rain came in the house.

Note: Sentence 2 is correct, but sentence 1 may also be acceptable.  As The Longman Concise Companion notes, "When two main clauses are very short, a comma is always appropriate but sometimes can be omitted, especially in public and informal contexts. . . .  Some academic readers, however, will expect you to use this comma in nearly every case" (433).