she become a doctor这句话中become为什么不是becomes

"become" is used in this sentence because it is in the past tense. The sentence is talking about something that has already happened in the past (she became a doctor), so we use the past tense of the verb "become." If we were talking about something that is happening now or in the future, we would use "becomes" instead. For example, "She is studying hard to become a doctor" or "She will become a doctor in a few years."