Non strappare il foglio del quaderno.

Because this is a negative command addressed to tu.

In Italian, the negative imperative for tu is normally formed with:

non + infinitive

So:

• Strappa! = Tear it!
• Non strappare! = Don’t tear it!

This is one of the most important imperative patterns in Italian.

Compare:

• tu: non strappare
• Lei (formal singular): non strappi
• voi (plural): non strappate

So non strappare is the normal way to say don’t tear when speaking informally to one person.

It is the infinitive form of the verb.

The basic verb is strappare, meaning to tear, to rip, or sometimes to tear out depending on context.

In this sentence, the infinitive is being used because, as mentioned above, Italian uses non + infinitive for a negative tu command.

So even though strappare is normally translated as to tear, here it functions as don’t tear.