REGRETS AND PAST POSSIBILITIES
A: He is sleeping during the lesson because he looks exhausted. What happened? B: (go to bed late) [possibility]
He might have gone to bed earlier, he could've gone to bed late
A: She is holding her broken phone and crying. What happened? B: (drop it on the floor) [possibility]
She might have dropped it on the floor.
A: He is running, but the bus has already left. B: (leave home earlier) [regret]
He should have left home earlier.