STUDENT:  1.I'm looking for my pencil.
CORRECT!

STUDENT:  2. I picked up trash.
CORRECT: 2. I picked up a/the trash.

Explanation:

a. We need to put articles on things that we don't need to put number on.

3.The criminal ran away from police.

STUDENT: The criminal ran away from police.
CORRECT: The criminal ran away from the police.
EXPLANATION:

a. We use the article "the" for organizations and system. Since "police" is an organization, we put  the definite article on it.