A: I don’t understand: he MUST be at home: the lights
are on, the car’s in the driveway, I can hear music, it’s his day off, it’s way
after closing time, and yet, he hasn't answered the door.
B: He can’t have heard the bell.
A: Oh, I haven’t tried the bell. What a good idea.
Statement of form
Talking about the present: modal
verb + base form
Talking about the past: modal
verb + have + past participle
What students need to know
This isn't talking about
facts; it's what's the speaker believes to be true – usually based on
evidence of some kind. He or she is either sure something is true (‘must’), or
sure something isn’t true (‘can’t’), or not very sure at all (“might” or
“could”).