Modals

Sentence: We ______ be quiet in the library.

• This sentence talks about a rule or strong obligation. In libraries, being quiet is compulsory, not optional.

• "Must" is used to express strong necessity, duty, or rule.

• Library rules require silence, so "must" fits perfectly.

Thus, the correct sentence is: We must be quiet in the library.

The other options are incorrect because:

A. mustn't:

• "Mustn't" means prohibition (something is not allowed).

• "We mustn't be quiet" means we are not allowed to be quiet, which is illogical here.

B. doesn't have to:

• "Doesn't have to" means no necessity.

• Also grammatically incorrect with the subject 'we' (should be "don’t have to").

C. doesn't:

• "Doesn't" cannot be used before a base verb like 'be' in this structure.

• Grammatically incorrect.