It depends on whether you are referring to undergraduate or graduate thesis defense.

If it is an undergraduate thesis defense, it depends on the requirements of your school. It is best to ask your senior. Different schools have different requirements, but in most cases, you will basically be allowed to pass. At least it won't be because of poor defense, but more often because of poor writing, such as plagiarism, or too many problems with language expression, which is not like English writing at all.

The general process is that you will have a few minutes to state the main content of the paper, and then the teacher will ask a few questions about your paper.

The usual questions are the meaning of the keywords in your paper, the significance of the research, etc.

For English majors, the key is not to be speechless. What you say must sound like English, which means your spoken English must be good. I would like to emphasize that for English majors, teachers value more your English foundation and will not and cannot force your scientific research abilities.

In short, get familiar with your thesis and practice speaking fluently. As long as the teacher agrees to you participating in the defense, it will be basically OK. If you want to be good or excellent, then you must perform well when answering. Some (mainly refers to good oral English).