having read the thread "He mustn't / couldn't have been hungry" He mustn't / couldn't have been hungry, I am still unsure whether or not there is such a wording as "mustn't have past participle".

Situation: I saw Peter and Jane holding hands. Speculation (my choice): They must have been lovers. The opposite of this would be: They can't/couldn't have been lovers. Incorrect: They mustn't have been lovers.

What is the situation with "mustn't have past participle"? Is there such usage, and if yes, in what context?

Many thanks for explaining this once again.


  • possible duplicate of 'Will' vs 'Must'. OP mustn't have checked very carefully for eisting questions on this topic, since I'm sure there must be many others besides that one. See negation of “must”, for example, which specifically addresses the "negation" aspect. Apr 18, 2014 at 21:23

3 Answers 3


When we say mustn't be, we mean cannot be. If we want to say that they either could or could not be, we will say don't have to be or needn't be or don't need to be.

You don't need to wear formal dress. You musn't wear formal dress.

The first one means that formal dress is optional; the second means that formal dress is forbidden.

So, to your examples. Can't/couldn't/mustn't have been lovers are basically equivalent.

I saw Peter and Jane holding hands. Since they were both married, they couldn't/can't/mustn't have been lovers.

These all mean roughly the same thing. Mustn't usually conveys a bit less forcefulness than can't or couldn't, but it is saying the same thing. For example:

He can't have been wearing a red tie. He hates red ties.
He must not have gone downtown this afternoon, since I saw him sitting on his porch.

The first is drawing a little bit stronger conclusion than the second, but they are both saying that the condition described is sufficient evidence to draw the conclusion.

Now, consider this:

Peter and Jane don't have to be lovers just because I saw them holding hands.

This is saying that the fact of holding hands isn't enough evidence to draw the conclusion that they are lovers. However, they might be.

Now, the simple past mustn't have a past participle. That means that you can not use a past participle with the simple past.

This, by the way, is the opposite of German. "Es muss nicht sein, aber es kann sein" translates "It doesn't have to be, but it can be." "It must not be" translates "es darf nicht sein."


"Mustn't" or "must not" has a very different meaning from "can't" or "couldn't" in this construction.

If you say:

Why were they holding hands? She can't have fallen for him!

there are two possibilities. You might know, for some reason, that she did not have feelings for him:

She can't have fallen for him; she is a robot, and not programmed to love!

Or you might simply be expressing disbelief in the possibility:

She can't have fallen for him; he's a disgusting slob!

The meaning changed if you use "must not" in this sentence (and "must not" is more idiomatic than "mustn't" in many dialects of American English; "mustn't" often comes across as archaic or fussy). "Must not" implies that you originally thought otherwise, but that you're accepting that your were wrong:

I saw them holding hands, so they must not have been enemies after all.

So in your example, it wouldn't make sense to say that since you saw them holding hands, they "mustn't have been lovers." That would imply that you thought they were lovers, but changed your mind after seeing them holding hands.

It would make more sense to say, "I always suspected Jonathan and Lois were having an affair. But I just found out that Jonathan is gay. So Jonathan and Lois must not have been lovers after all."


You mustn't have understood when you were studying modals!

I think you're looking at this the wrong way. Let's start with a simple present perfect example. He has arrived. That sounds like a certainty and it implies the speaker knows for sure. But if there is no certainty, if it's a guess, then we insert a modal such as must or should into our sentence. He must have arrived. Now we have a supposition because the speaker doesn't know for sure. The tense remains the same, it is indicated by "has arrived", a present perfect construction. The modal does not affect the tense, it does not conjugate like other verbs, it just switches the attitude from certain to likely.

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