Modals with "not"

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]

Originally Posted by Omarmusa123

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.