I have a question regarding modals: must have/had to.
When an author of a book, that's written in past tense, chooses to use modal "must" he has to use its past tense form:
The car crashed. The roads had to be slippery. (The roads were slippery)
When he writes about something that happened even before, he'd write:
The car had crashed. The roads had to have been slippery. (The roads had been slippery)
Why does it look so different in present tense(?), "had to" and "must have" even mean slightly different things. Why one cannot replace:
That game must have been really bad. (The game was bad)
That game really had to be bad. (The game was bad, in past tense. A weird sounding assumption, when used in present tense.)
What is the difference?