Question

why don't you look out of the window? write into assertive sentence

Answer 1

You must look out of the window
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hope it help you

Answer 2

If we turn this integrative sentence to an assertive sentence ten we need to write “You should look out of the window.”

Assertive sentences are also known as the declarative sentences asserting or declaring a statement.  

The assertive sentence always ends with a full stop at the end.  

In English, almost all sentences are of certain type except when some special thoughts are written.