The Imperative Sentence

An imperative sentence gives a direct command to someone -- this type of sentence can end either with a period or with an exclamation mark, depending on how forceful the command is:

Sit!

Read this book for tomorrow.

You should not usually use an exclamation mark with the word "please":

Wash the windows!

Please wash the windows.

Normally, you should not use imperative sentences in academic writing. When you do use an imperative sentence, it should usually contain only a mild command, and thus, end with a period:

Consider the Incas

Definition of an imperative Sentence

•  An imperative sentence:
• is used to give a direct command to someone
• can end with a period or an exclamation mark depending on the degree of the command
• containing the word "please" should end with a period
• is commonly used in speech but not often in writing

Examples of Imperative

1.“Eat your spinach!” is an imperative sentence.

2.“Help” in the sentence “Help me!” is an imperative verb.

3.a verb in the imperative mood

4.People resented his imperative tone of voice.

5. I have begun to feel each time as if I am mutilating my antennae (which is how Rastafarians, among others, think of hair) and attenuating my power. It seems imperative not to cut my hair anymore. —Alice Walker, Living by the Word, (1981) 1988

Reference
http://www.merriam-webster.com/dictionary/imperative
http://funeasyenglish.com/american-english-grammar-imperative-sentence.htm