SUBJECT QUESTIONS:
A subject question asks about the subject of the sentence. Do NOT change the word order of the question.
Subject questions normally begin with who, which, what. In subject questions the verb is in the affirmative form.
Who called Marina?
Who gave you this key?
OBJECT QUESTIONS:
An object question asks about the object of the sentence.
You MUST change the word order of the question. Object questions normally begin with who, which, what. In object questions the verb is in the interrogative form.
Who are you talking to?
What did he say?