Must and can't

You must be insane! Must is an auxiliary verb to talk about something we believe it's certain. For example: A lot of people have been playing this game, it must be very interesting. It is only my prediction about the game, I haven't played it yet. 

In contrast, can't is an auxiliary verb used to talk about what we believe it's impossible. For example: You have eaten in the morning, you can't be hungry. 

Must have done and can't have done are used in the past. There are other forms of must and can't like: must be doing, must have been doing, can't be doing, can't have been doing. For example: The man has been behind me for 20 minutes. He must be following me. Or The boy was standing outside the bookshop when I came out. He must have been waiting for somebody.