if the sentence is positive, the tag question is negative
He is very handsome, isn't he?
He was late, wasn't he?
If the sentence is negative, the tag question is positive
They aren't very nice to you, are they?
They weren't late, were they?
With modal verbs, the tag questions are always the same
He can't see you, can he?
With other verbs, you use do/did, in the past you use did and in the present you use do.
He volunteered yesterday at the shelter, didn't he?
She volunteers today at the shelter, doesn't she?