20. These three kinds of sentences are called assertive, interrogative and imperative.

An assertive sentence states a fact or an opinion.

An interrogative sentence asks a question.

An imperative sentence gives a command, makes a request or expresses a wish.

21. Any of these three kinds of sentences may be exclamatory; that is, it may express surprise, excitement, impatience, or some other emotion. For example:

This is an assertion expressed as an exclamation.

Here we have a question in the form of an exclamation.