Phrase vs Imperative sentence.

Phrase is a group of words that do not contain a subject and a verb. 

On the bus, On the train, at the traffic light and so on.

When you give an order or command, use the base form of a verb, such as give, read or come. This base form is called the imperative.

For example; Open the book, Switch off the light, Turn off the heater and so on.

Imperatives are very direct. To be more polite, you can use please before the imperative.
Please show me your mobile.

Please call me later.

To make negative imperatives, use do not or don’t before the base form of the verb. Do not bring calculators into the examination hall. Please don’t change anything on my computer.