你真是个好老师用英文怎么说?

you are a really good teacher.

really是副词，修饰good形容词。

You are a good teacher!

You are real a good(或kind) teacher.

you are really a good teacher

What's the reason?Please tell me.
It's none of reason for it.This answer could be true!

What a good teacher you are!(感叹佝）