couldn't have VS mustn't have

[ian_k]

He mustn't have been very hungry.

He couldn't have been very hungry.

I am not a native speaker either.
But to me the first could mean :
I am certain he wasn't hungry.
(Similar to a sentence: "He must have not seen it coming")
The 2nd line would mean:
It's impossible that he was hungry.

Similar situation:
Well, he must not be the thief they're looking for then. (We'd better let him go)

Well, he can't be the thief. (He wasn't anywhere near the victim during that time. You can't arrest him.)

[mmasny]

I am certain he wasn't hungry.

It's impossible that he was hungry.

Similar situation:
Well, he must not be the thief they're looking for then. (We'd better let him go)

Well, he can't be the thief. (He wasn't anywhere near the victim during that time. You can't arrest him.)

I don't understand what difference you have in mind. Can you give further explanation?

[Raymott]

I am not a native speaker either.
But to me the first could mean :
I am certain he wasn't hungry.
(Similar to a sentence: "He must have not seen it coming")
The 2nd line would mean:
It's impossible that he was hungry.
But "I am certain he wasn't hungry." and "It's impossible that he was hungry." mean basically the same thing - "According to me, he could not have been hungry."

Similar situation:
Well, he must not be the thief they're looking for then. (We'd better let him go)

Well, he can't be the thief. (He wasn't anywhere near the victim during that time. You can't arrest him.)
Again, these sentences mean basically the same thing.
If he can't have been the thief, then he mustn't have been the thief.
Somebody else must have been the thief.

The use of these two phrases has been discussed. It has been discovered that they can mean the same as each other in a certain context; but can also have different meanings if used differently.

[ian_k]

I don't understand what difference you have in mind. Can you give further explanation?

Yes, they're basically giving the same argument. But one stresses on the certainty of the speaker's own claim, while the other on the impossibility.

Here's a situation:
"I waved at your wife but she kept on walking, she must not have seen me (i'm sure she didn't see me)."
"Well, she couldn't/can't have (of course that's impossible for her to see you), because you were waving at her from the third floor."