There are four basic types of main clause: declaratives (statements), interrogatives (questions), imperatives (orders/instructions) and exclamatives (used for exclamations).
In the examples below, x is any other element in the clause (e.g. object, predicative complement):
Declarative clauses most commonly function as statements. The usual word order is subject (s) + verb (v) + x. Declaratives can be affirmative or negative. They make statements about how things are and how they are not.
[S][V]I saw[X]them last week.
[S]I[V]didn’t see[X]them last week.
[S]Some courses[V]begin[X]in January.
[S]Some courses[V]don’t begin[X]until March.
Sometimes we use declaratives as questions or requests:
Those are the only tickets left? (question)
Yes, just those two.
You could pass me the spoon. That would be helpful. (request)
Interrogative clauses most commonly function as questions. The usual word order is (wh-word) + auxiliary/modal verb (aux/m) + subject + verb + x:
What [AUX] [S]are you[V]doing?
[M]Can[S] [V]I come[X]with you?
Interrogative clauses can be affirmative or negative.
Imperative clauses most commonly function as commands, instructions or orders. The usual word order is verb + x. We do not usually include the subject in an imperative clause. We use the base form of the verb:
Come on. Hurry up!
Leave me alone!
Put it in the microwave for two minutes.
Imperative clauses can be affirmative or negative. We make negative imperatives with auxiliary verb do + not. The contracted form don’t is very common in speaking:
Leave the door open.
Don’t leave the door open.
Don’t be sad.
We use do not in more formal contexts:
[instructions on a jar of coffee]
Do not make coffee with boiling water.
We can use the short form don’t as an imperative answer, or as a reaction to something:
Shall I open the window?
No, don’t. I’m freezing. (No, don’t open the window.)
Imperatives with subject pronoun
Sometimes we use you (subject pronoun) with an imperative clause to make a command stronger or to strengthen a contrast. It can sometimes sound impolite:
Don’t you ever read my letters again.
[talking about washing up dishes]
You wash, I’ll dry.
In informal speaking, we can use an indefinite subject (e.g. someone, somebody, no one, nobody, everyone, everybody) with an imperative:
No one move. Everyone stay still.
We often use an imperative to make an offer or invitation:
Have some more cake. There’s plenty there.
Imperatives with do
We sometimes use do for emphasis in an imperative clause, especially if we want to be very polite:
Do sit down, please.
Imperatives with let
In speaking we usually use let’s for first person plural imperatives (us) to make a suggestion. In more formal situations we use let us:
Let’s go and eat.
Now, let us all get some sleep. (more formal)
For third person imperatives (him, her, it, them) we form an imperative clause with let:
Mr Thomas is here to see you. Shall I send him in?