Aslam ________ the door before entering the room.

Knocks

Has knocked

Had knocked

Knocking

all are correct

This is a sentence of Past Perfect Tense.

Had is the helping verb of Past Perfect Tense.

Aslam had knocked the door before entering the room.

اسلم کمرے میں داخل ہونے سے پہلے دروازہ کھٹکھٹا چکا تھا۔

agm.04.03.23