INCORRECT: It doesn't excuse you acting like this. CORRECT: It doesn't excuse your acting like this. (You can also say "behaving.") CORRECT: Our friendship ended after that. INCORRECT: Who knows what might have happened if you didn't come? CORRECT: Who knows what might have happened if you hadn't come? INCORRECT: There wasn't one month I never got the rent on time. CORRECT: There hasn't been one month that I haven't got the rent on time.