what it the complete ssubject in this sentence... Can you explain last night's homework to me?

The simple and complete subject is "you."

Thank you. That's what I was thinking but didn't think it could be just that.

The complete subject in the sentence "Can you explain last night's homework to me?" is "you."

To identify the subject in a sentence, we need to ask "Who or what is performing the action or being described?" In this sentence, the action being performed is "explain," and the subject is the one who is doing the explaining, which is "you."