English grammar

Can I use simple present to tell actions that happened in the past?

I saw in the movie the character asked " do you know him? " and others said " yes I know him"

the action "know" it happened in the past so why they don't say "did you know him?" and "I knew him"

Also, I can't distinguish when we use "DID you know him?" and "DO you know him?"

please help me.