A practical english grammar - Vyssaja skola 1978

The modals in negative, interrogative, and echo construction
Sound symbols

The modals behave in these constructions just like other auxiliary verbs. In questions, they are placed before the subject; other parts of the predicate remain in their usual place.

Can you repair this watch?

Must I wear my glasses all the time?

In the negative, they combine with n’t (with a few exceptions in American English) or they are followed directly by not.

Can’t you stay longer?

We couldn’t find the key.

They won’t be here today.

It will not be possible for me to finish the work this morning.

CONTRACTIONS OF MODALS WITH N’T

can    can’t

could    couldn’t

will    won’t

would    wouldn’t

should     shouldn’t

must    mustn’t

The forms mayn’t, mightn’t, and shan’t exist, but they are rarely used in American English. Even speaking informally, most speakers say may not and might not. Shan’t is avoided in favor of shall not.

In echo constructions, the modal stands alone, to replace an entire predicate.

Can they speak English as well as you?

Yes, they can.

Modals as echoes occur frequently in tag questions.

We can’t park here, can we?

They won’t arrive today, will they?

The children must wait outside, mustn’t they?