Compare these sentences:
Who does Joe love? Joe loves Bella.
Who loves Joe? Bella loves Joe.
OBJECT QUESTIONS
We form object questions with an auxiliary or modal verb by inverting the word order.
He is a fireman. → Is he a fireman?
Where there is no auxiliary or modal verb, we use do to form the object question instead.
You know him. → Do you know him?
SUBJECT QUESTIONS
Subject questions are formed by using the question word as the subject of the sentence instead. We do not use an auxiliary verb and we do not invert the word order.
Object questions ask about the object of the answer:
Who did you see? I saw Jackie. (Jackie = object)
Subject questions ask about the subject of the answer:
Who saw Jackie? I saw her. (I = subject)