How to say "I thought you are..." to a response?

Question

Let's start with an example:

A: 你是学生吗？

B: 不是，我是老师。

C: I thought you are student!

_{Note that C is a different person from A.}

In this case, how can I say "I thought..."?

I know in Mandarin, you usually use an adverb or an adverbial phrase to express the past tense (e.g. 以前), but in this case, I don't know how I should express the past tense. At least I feel it is awkward to say 以前我认为你是学生.

Or maybe I should add 过 to say 我认为过你是学生, but I'm not sure and still feel a bit awkward...

If I say 我认为你是学生, I feel like the person C thinks the person B is lying and is actually a student. This may lead to the misunderstanding, I think.

So how should I express the past tense in these cases?

Answer 1

It is "我还以为你是学生"

认为 (consider; think) can be in any tense

but 以为 already mean "(previously) thought"

还 is a common adverb with many meanings. In this case, it means 'even'

还以为 = '(even) thought'

Answer 2

You need the subjunctive 'were' for this sentence in English, because the apparent state is not real, or you would not have said this.

I thought you were a student.
我本来以为你是个学生。

Answer 3

The idiomatic ways in this situation:

1. 我(还)以为你是学生呢！
2. 之前，我还以为你是学生呢！

还 can be omitted. If you want to put an exclamation mark at the end, you'd better add 呢.