Degrees of certainty: Past time

Examples:

Past Time: Affirmative

--Why wasn't Marry in class?

(a) 100%: She was sick.
(b) 95%: She must have been sick.

(c) Less than 50%:
She may have been sick.
She might have been sick.
She could have been sick.

In (a): The speaker is sure.
In (b): The speaker is making a logical conclusion; e.g., "I saw Mary yesterday and found out that she was sick. I assume that is the reason why she was absent. I can't think of any other good reason."
In (c): The speaker is mentioning one possibility.

Past Time: Negative

(d) 100%: Sam wasn't hungry.

(c) 99%:
Sam couldn't have been hungry.
Sam can't have been hungry.

(f)  95%:  Sam must not have been hungry.

(g) Less than 50%:
Sam may not have been hungry.
Sam might not have been hungry.

In (d): The speaker is sure.
In (e): The speaker believes that it is impossible for Sam to have been hungry.
In (f): The speaker is making a logical conclusion.
In (g): The speaker is mentioning one possibility.



Cheers!
Naeem