There's a mistake. I put "I don't know what to do to become a nurse" and it wasn't accepted, but "I don't know what to do to to become a nurse" (with two "to's") was accepted.
Duolingo sometime @@
By this time in the course, I have seen this sentence so many times that I don't need to read it or even attempt to translate it. I read "Tôi không biết..." and I know the rest of the sentence.
Come on Duolingo. Wake up. Teach me something new.