The imperative tense in English is used to give an order, a warning, an appeal, an advice, a suggestion, an instruction and in some cases a request to another person, to a group of people or to animals. It is very easy to form the imperative sentence: Simply take the verbs' infinitive form (without the "to" infinitive indicator). Usually the verb will be placed at the beginning of the sentence.

For example:
Close the door.
Give me the keys.

The above form is used if a person gives an order to another person or to a group of people. But if the imperative includes oneself in the group of people who receive the order the words "Let us" or "Let's" are added before the infinitive in the sentence.

For example:
Let's eat something, we are all hungry.
Let us be happy, we are all fine.

In order to create a negative form of an imperative, place "do not", "don't" or "let's not" in front of the infinitive form of the verb (again without "to").

For example:
Don't go, I need you here.
Let's not be sad, we are all fine.

The imperative form can also be used to make a request, if you add "please" to the imperative sentence. You can also add "Would you" instead or in addition to "please" (which can be a question or a polite order).

For example:
Please come, I need you there.
Please don't go, I need you here.


More examples:
Stand up when I speak to you (order)
Press the button in order to activate the machine (instruction)
Do not touch it, it is hot! (warning)
Take a left at the corner (advice, suggestion)