Which best identifies the tense of the verb taught?

Taught is in past tense, when you say he taught it means he has already done it

The word 'taught' is in the past tense because you already taught them. If it was in the present tense it will be 'teaching' because you are teaching them now, not in the past and not in the future. If it was in the future it will be 'to teach' because you are going to teach that person, you are not teaching now at this moment and u haven't taught them already.