Student or Learner
_"have you known about Ann's new boyfriend?"
_"Yes,I've,but I forget(or have forgotten) to ask her his name."
Which tense should I use in the second sentence?(past simple or present perfect?)
But if I want to use"present perfect",which of them will be correct?
If you wish to be correct, you should not try to use present perfect.
Did you meet Ann's new boyfriend?
Yes, but I've forgotten his name. -- This is okay. You knew it. You no longer knew it. But you can learn it again. You have forgotten it.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.