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.