First of all, you should know that the teacher will not let you pass unless it is particularly outrageous.

As long as you do your thesis carefully, you should have something to say to the teacher's questions without answering them. Besides, teachers don't know everything. They also ask questions they know.

Talk to your tutor before you reply, and he will give you some information.

Also, take a deep breath when you are nervous in your defense, or directly tell the teacher that you are nervous, which can also eliminate the tension.