"I think Mr. Tanaka will quit the company soon."
How would I say "Mr. Tanaka thinks you will quit the company soon." (Context: I want to know if my colleague is really going to quit.)
I am asking this because we know where the quote ends (on と), but where does it begin? If it's implied to start at the beginning of the sentence, I have no idea how i would say such a thing.
Using 思います (omoimasu) in the -masu form refers specifically to your own thoughts. When saying what you think other people think, you use 思っています (omotte imasu). ThoughtCo
田中さんはあなたがすぐに会社を辞めると思っています (tanaka-san wa anata ga kaisha o yameru to omotte imasu), where あなた would probably best be replaced with the person's name, should convey your translation.