Let's start with an example:
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?