couldn't have VS mustn't have

[Kendama]

I always have trouble teaching the difference between these two to show degrees of negative certainty.

For example, is there much difference between these:

He mustn't have been very hungry.

He couldn't have been very hungry.

Thanks.

[Tdol]

I'd use the second to contradict or emphasise.

[Raymott]

I always have trouble teaching the difference between these two to show degrees of negative certainty.

For example, is there much difference between these:

He mustn't have been very hungry.

He couldn't have been very hungry.

Thanks.

I think meaning is more important than trying to rank modals in order of possibility/certainty, etc.

These sentences can mean completely different things.
He mustn't have been hungry - because he didn't eat what we gave him.
He couldn't have been hungry; he had just eaten a seven course meal.

As far as ranking modals, I believe there are are three basic levels of probability.
No possibility at all: He won't come.
A possibility: He [might, may, could] come.
Certainty: He will come.

Any other distinctions need to be done with adverbs.

[mmasny]

He mustn't have been hungry - because he didn't eat what we gave him.
He couldn't have been hungry; he had just eaten a seven course meal.

Can't the second one mean that he didn't eat what you gave him too?

[mara_ce]

He mustn't have been very hungry.

He couldn't have been very hungry.

I’m not a native speaker but I usually teach this type of things.
In my opinion, the first one doesn’t make sense. I’d say:

He can’t have been very hungry. (couldn’t have is possible instead of can’t have)

Must have and can’t have are used to make deductions about past actions.

I must have left my wallet in the car. (I’m sure I did)
Jim can’t have noticed you. (I’m sure he didn’t)

To express possibility or uncertainty about past actions you can use might have, may have and could have.

Jean might have missed the train. (Perhaps she did)
He may not have received the letter. (Perhaps he didn’t)
You could have been killed! (It was a possibility)

[billmcd]

Yes.

[Raymott]

Can't the second one mean that he didn't eat what you gave him too?

Yes, it's ambiguous; and that is partly why I say that they shouldn't be seen as merely differences in degree, and why you need adverbials to discriminate meanings.

[Raymott]

I’m not a native speaker but I usually teach this type of things.
In my opinion, the first one doesn’t make sense.
It does make sense to a native speaker. It means the same as "He can't have been very hungry."

Must have and can’t have are used to make deductions about past actions and states.
That's right. And since he didn't eat the food we gave him, we deduce that "He mustn't have been very hungry" or that "He can't have been very hungry".

R.

[mara_ce]

R.

Thank you, I’ve learned something new! I had never seen 'mustn’t' associated with certainty. My grammar books relate it to prohibition.

[Raymott]

Thank you, I’ve learned something new! I had never seen 'mustn’t' associated with certainty. My grammar books relate it to prohibition.

Yes that's true, but you can't prohibit something that has already either happened or not happened in the past. That frees up mustn't for a different meaning in the past. "Must have" is used more commonly than "mustn't have" for the past.

I'm sure you've heard of this situation:
Peter: Where is Mary? She was supposed to be here an hour ago.
John: She must have missed her bus.

This doesn't mean "She had to miss her bus". It means "I deduce that she has missed her bus."
Similar principles apply to "mustn't" in the past.

Grammar books rarely cover the modal verbs comprehensively because there is just too much you could write about them.