Hello Hossein, There are several things you can do with that sentence, and it all depends on meaning and context. Here are some examples. I thought you were a teacher. First lets look at meaning. 'Were' is 'to be' in the subjunctive mood. Subjunctive is used to talk about something that isn't true. So if you say this, it means that the person is not a teacher or you think that they are not a very good teacher. Now let's add some context. Let's say that you are describing something in the past. Bob has just met Sue, and they are talking about their careers. They live in the same town, so Bob has seen Sue before. Bob: "I saw you on the university campus. I thought you were a teacher." Sue: "No, I'm not. I was there to attend a seminar." Here is another context. Principal of the school: "Your students are all failing and I am receiving many complaints from parents. Why can't you handle this? I thought you were a teacher!" (The speaker is saying that the teacher is not a good teacher. This is a rude thing to say.) You could also write the sentence this way: I think you are a teacher. This is in present simple and states what the speaker thinks. Bob: Can you guess my job? Sue: Hmmmm....I think you are a teacher. Bob: That's right! How did you guess? You could add a little more information to tell someone what kind of teacher they are. Marvin: "Thank you for explaining that to me, Mr. Smith. I think you are a great teacher!" Mr. Smith: "You're welcome, Marvin. I'm glad I could help."

"Is this sentence correct?" I thought you "were" a teacher. You need to use "were" because you used the past tense at the beginning of the sentence.

I thought you were a teacher.