I would argue that might is not for ability, it is for possibility. For ability, we also use can a lot in the present. For example: I think I could/can pass the exam, but I might not. We can win if we play well, but we might not. I don't know if he can come to the party, but I think he might. In the past, we use "could have" to show that you had the opportunity to do something but (probably) didn't. We don't use might here at all. For example: I could have passed the exam, but I didn't. I could have gone to the match but I didn't buy a ticket. We use might to talk about possibility in the past when we don't know what has happened (We are ignorant). We can also use could have in this way too. Q: Why did John miss the party? A: I don't know, he might have been busy. (He could have been busy) Q: Why did the crash crash? A: The driver might have gone too fast. (He could have gone too fast) Here are a couple of resources I made that might help. https://askpaulenglish.wordpress.com/2016/09/22/mustmight-have/ https://askpaulenglish.wordpress.com/2017/08/05/could-have-3/

With all of those sentences, you can use "could" with them alone depending on the context of the conversation (You could behave well, You could have been behaving well during the party, etc). "With "might" you usually need a reason for the use of "might." "If you stop eating so much sugar, you might behave well during the party." You can also switch it: "You might behave well during the party if you stop eating so much sugar." "Might" is usually used with suggestions to a person on what they should do and why they should do it. "Could" also can be followed by a reason, but depending on the context of the discussion it can be alone. I hope these links help with your infinitive questions: http://www.ef.edu/english-resources/english-grammar/continuous-infinitive/ http://www.ef.edu/english-resources/english-grammar/perfect-continuous-infinitive/ I hope this helps you! :)