MODAL VERBS OF PROBABILITY
Sometimes Modal auxiliary verbs can express probability, or how certain a situation is.

Can't= logically improbable
We're not absolutely sure but we're pretty certain
(Ex. She can't have a ten-year-old daughter! She's only 24!)

The continuous infinitive is formed with be + ing
(Ex. You must be joking
I could be sitting)

֍ Probability in the past
The perfect infinitive is formed with have+past participle.
(Ex. He must have caught a later train)

The continuous infinitive is formed with have+been+ing
(Ex. She must have been joking
They can't have been trying very hard
He could have been lying to you)