Question

this is girl she is a doctor这句话英语写的对吗？

Answer 1

a girl.

This is a girl. She is a doctor.

这是个女孩，她是医生。

girl是可数名词，前面要加冠词a, 一个女孩。

Answer 2

this is girl she is a doctor 如果中间没有逗号，那么就应该用连接词 and或变为定语从句。如：
This is a girl and she is a doctor. 或者：
This is a girl who is a doctor.