During a court hearing, can the defendant ask the plaintiff questions?

Not unless he is representing himself, and there is a saying "A man who represents himself has a fool for a client". Generally, your best bet is a good lawyer who can ask the questions.

During a court hearing, can the defendant ask the plantiff questions?

Yes, the defendant, or the defendant's attorney if he has one, can cross-examine the plaintiff if the plaintiff testifies during the presentation of plaintiff's case. If plaintiff has rested his case without testifying, defendant can call the plaintiff as a (hostile) defense witness.

All this assumes, of course, that the defendant's testimony is relevant.