How to say "I thought you are..." to a response? 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 &quot;I thought...&quot;?
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?
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'