Modals with "not"

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Anonymous

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Is there a rule for the word that "not" negates?
e.g. it negates the infinitive "smoke" in "You mustn't smoke," but the modal "could" in "She couldn't have fallen asleep."
I need an answer that can make my students understand rather than memorise the meanings of these constructions.
 

Casiopea

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Omarmusa123 said:
Is there a rule for the word that "not" negates?
e.g. it negates the infinitive "smoke" in "You mustn't smoke," but the modal "could" in "She couldn't have fallen asleep."
I need an answer that can make my students understand rather than memorise the meanings of these constructions.

You smoke.
You do not smoke ~ You don't smoke.

Note, even though not contracts with the nearest verbal (e.g. do, must, could, etc.) it negates the act (i.e. smoke).

You must smoke.
You must not smoke ~ You mustn't smoke.

You could have fallen down.
You could not have fallen down ~ You couldn't have fallen down.
 
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