SPECULATION AND DEDUCTION

SITUATION: I was at home, eating some pop corns while I was watching a movie, then, my mother calls me, I went to the kitcken, talk with my mon, and then I return to the livingroom. But I notice that the movie that is on TV. is "Tangled" and I'm sure that I was watching "Shrek", so I don't know what happend. I decided to continue watching the movie, but when I want some pop corns, I notice that there's nothing in my plate. It was rare, but maybe I eat all the pop corns and I didn't remember. But something is sure, my dog couldn't eat my pop corn because he doesn't like it.

I'M SURE THIS HAPPENED

SUBJECT + MUST + HAVE + PAST PARTICIPLE

I'M NOT SURE IF THIS HAPPENED

SUBJECT + MIGHT/MAY/COULD + HAVE + PAST PARTICIPLE

I'M SURE THIS DIDIN'T HAPPEND

SUBJECT + CAN'T/COULDN'T + HAVE + PAST PARTICIPLE