I'm learning about the usage on (으)세요, but my textbook explains that you cannot use 이다 in (으세요) form. So the following sentence is invalid:
However, the textbook says the correct form is this:
But I don't know why you must use 가 there. Because the sentence means "(You) Become a doctor.", where a doctor is an object, I wonder why the sentence use 를, like:
Or 에, to mean the subject is going to.
For me, the sentence looks like a doctor is a subject, not an object...
The example sentence comes from a book Korean Grammar in Use (Beginner), Unit 7, Page 167.