Chapter 9-4-5: 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:
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.