"I thought you were a doctor."
Translation:Eu pensei que você fosse um médico.
I would agree with you considering the context of the sentence. I used pensava. Basically, I thought "all this time" that you were a doctor. I guess achei or pensei would be more like "I thought in that moment (that I met you) that you were a doctor". I guess it doesn't make it clear so Duolingo accepts both answers as correct.
The subjunctive in English is quite invisible, but it still exists. In the subjunctive form, "to be" becomes "were":
- If I were you = Se eu fosse você
Normally, many people use "was" instead of "were".
But this specific sentence uses only "were" because it's "you", not "I".