Scope of Negation

With verb phrases that contain modals, the modals are sometimes included within the scope of negation. The negation extends throughout the verb phrase. This is despite the fact that the modals always precede the adverb not or its contracted form. 

In the following sentences the modals are within the scope of negation:

David won't come to our party.            

Vanessa may not attend the concert.

The first sentence means that David has no desire to go to the party. In other words, he does not accept the invitation. In the case of the second sentence, Vanessa does not have permission to attend the concert. Maybe her parents have told her that she must stay home and study.

However, in the following sentences, the modal is not within the scope of negation. Only the meaning of the main verb is negated. Consider the following sentences:

I won't tell your brother.

Andrew might not pass the exam.

In the first sentence, the speaker promises not to tell the hearer's brother. In other words, the speakers agrees to the hearer's request. This is the opposite of the previous example in which the speaker declines the invitation to the party.

The second sentence expresses the possibility that Andrew could fail the exam. Here the modal might could also be replaced with could and may. It is not certain that Andrew will pass.

The sentence Vanessa may not attend the party is ambiguous. It has two possible interpretations. One is that Vanessa is not allowed to attend the party. In this interpretation, the modal is within the scope of negation. However, if may expresses probability, Vanessa may not attend the party has the meaning of Vanessa might not attend the party. In this case, the modal is outside the scope of negation.

The scope of negation does not always include modals in verb phrases. When the modal may is within the scope of negation, the meaning expresses prohibition. However, when the modal may is not within the scope of negation, the meaning expresses the probability that the intended event will not happen. Modals in verb phrases are sometimes within the scope of negation, and sometimes they are not.