Didn't is the everyday short form of did not. The apostrophe marks the missing o of not: did + n't. Use it in speech and in friendly writing: She didn't come. I didn't see you. In very careful or very formal writing, people may still write did not in full; the meaning is the same, and the short form just sounds like talking. It belongs with the past: you are saying that something was not true, or did not happen, before now. You cannot pair it with a present verb (not didn't goes). The n't piece is the same short not you already know from don't and isn't.