***** NOT A TEACHER *****
1. IF I understand my books, we can parse your sentence like this:
The last film (that you saw) + was + what.
The last film that you saw = complete subject.
was = the linking verb.
what = a pronoun. In this case, an interrogative pronoun because it starts the question.
What is your name? For the sake of analysis: Your name (subject) + is (linking verb) + what (interrogative pronoun).