Поставьте глаголы в скобках в past indefinite, past continuous и past perfect. when i (hear) his knock i (go) to the door & (open) it but i (not/ recognize) him at first because i (not/ wear) my glasses.

When I heard his knock I went to the door to open it but I didn't recognize him at first because I wasn't wearing my glasses.

When i had heard his knock i went to the door to open it but i didn't recognize him at first because i was not wearing my glasses