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What is the negative past form of "must" that means prohibition?

For example, in "The room was full of gas, he _______ a match", if I fill in the black with "didn't have to light" it would mean a lack of necessity, not prohibition, wouldn't it? And with "must not have lit", it would mean strong negative probability.

What is the correct way to express strong past negative prohibition using "must"?

Thank you in advance.
Comments  
Hi shcho23

What about 'could not'? It's a strong no-no. I was thinking of 'must not' but then I was quite sure if you could use it with the past tense.

PBF
I wish I could edit my reply. I meant to say '...I wasn't quite sure...'
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Thank you for the reply.

Is it so because "must not" has the same meaning with "can not"?

But if I say "could not", how do I tell the difference between "wasn't able to" and strong negation?

Is it contextual?
Must not and cannot have different meanings, but must has no past, future, infinitve, participle, etc, and so we must substitute it with something else, usually could not. For strong negation simply use a different phrase:
The room was full of gas; lighting a match was out of the question.
There was no way he could light a match with the room so full of gas.
He absolutely couldn't light a match in a room full of gas.

Yes, if you use "could not" the difference is often contextual. Consider these two sentences:
He couldn't light the match; the wind was too strong. (wasn't able to)
He couldn't light the match; the room was full of gas. (negation)
In spoken english, we would emphasise "light" in the first sentence, and "couldn't" in the second.
I would second Anon's suggestion that you use a different phrase.

Prohibitions in the past are difficult. The speaker can't impose a prohibition, as the moment has now gone. Therefore the prohibition can only have existed in the thoughts or words of one of the people involved:

1. The room was full of gas. He knew he shouldn't light a match. ["I mustn't light a match!"]

2. The room was full of gas. MrQ told him he shouldn't light a match. ["You mustn't light a match!"]

Best wishes,

MrP
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shcho23What is the negative past form of "must" that means prohibition?
There is no such form. You have to paraphrase a bit, as suggested above.

The room was full of gas; he couldn't light a match.
...; it was not possible (for him) to light a match.
...; it was not advisable (for him) to light a match.

...; he dared not light a match.
...; he had to be sure not to light a match.

...; it would have been foolish to light a match.
...; lighting a match would have been crazy.

...; he knew (that) he mustn't light a match.

In the last case, you let the verb knew carry the past tense idea. must not (mustn't) in a subordinate clause, as shown there, is OK.

CJ
musn't or must not