§ 17. Imperative sentences express commands which convey the desire of the speaker to make someone, generally the listener, perform an action. Besides commands proper, imperative sentences may express prohibition, a request, an invitation, a warning, persuasion, etc., depending on the situation, context, wording, or intonation.
Stand up! Sit down. Open your textbooks.
Formally commands are marked by the predicate verb in the imperative mood (positive or negative), the reference to the second person, lack of subject, and the use of the auxiliary do in negative or emphatic sentences with the verb to be.
Commands are generally characterized by the falling tone, although the rising tone may be used to make a command less abrupt. In writing commands are marked by a full stop or an exclamation mark.
A negative command usually expresses prohibition, warning or persuasion.
Don’t cross the street before the light turns to green.
Don’t allow children to play with matches.
Commands can be softened and made into requests with the help of the word please, the rising tone, a tag question or a “yes-no” question beginning with will or would.
Speak louder, please.
Repeat the last word, will you?
Would you do me a favour?
The falling tone and an exclamation mark at the end of a sentence opening with will express irritation and impatience, as in:
Will you stop arguing!
Will you be quiet!
§ 18. Though in the vast majority of commands the subject is only implied, the subject expressed by the pronoun you occurs when it is necessary (a) to specify the subject, sometimes for the sake of contrast; (b) to convey the speaker’s personal attitude to the event presented in the sentence (for example, irritation, anger, threat, impatience); (c) to soothe somebody. The subject in these cases is heavily stressed.
a) You come first, and I’ll wait a little.
You come first, and he will have to wait.
b) You say it again, and I’ll turn you out of here!
Just you wait, Mr Higgins.
c) You be a good girl, and don’t worry.
Note the initial position of the operator in negative commands with a subject.
Don’t you interrupt me.
§ 19. In the case of first person plural and third person singular and plural subjects, the imperative let is followed by a personal pronoun in the objective case.
Let him try again.
Let them come in.
Let us have some tea.
A first-person command often implies invitation or suggestion and may be followed by the tag shall we.
Lets do it together, shall we?
There are two negative constructions with let for the first person.
Let’s not quarrel about trifles.
Don’t let’s quarrel about trifles.
A third-person command admits of only one negative construction:
Don't let him interfere in our affairs.
A third-person command may begin with a noun or a pronoun denoting the person addressed.
Somebody switch off the light.
Mary and John fetch dictionaries.
Here the corresponding negative is:
Don’t anybody switch off the light!
§ 20. The imperative of some verbs may acquire interjectional force. Thus the forms listen, look (here), see (here) (Am.) - are used to attract attention.