Present tense: mv + verb
We know it's true (95-100% sure):
Thanks for cooking! I must say that
this is delicious.
Something that's possibly true (50-50%):
Going to a Christmas market might be the best holiday activity ever.
Something we know isn't true:
A ten! That can't be right because I
made a mistake.
Past tense: mv + have + past participle
Something we know was true:
You were in an accident? That must have been scary.
Something that was possibly true:
Sorry, mom. I may have eaten the last piece of chocolate.
Something we know wasn't true:
You saw Ms. Dangerfield on The Voice?? It couldn't have been her, she can't sing.