“What happened (to my father)?”
This is a subject question because “what” (the question word) is the subject of the verb “happened”. The subject questions don’t take auxiliary verbs
Below are subject questions. The verb and its subject are bolded and italicized. The speaker is expecting the answer to be the subject of the sentence:
• What killed him? – A croc. (A crocodile killed him.)
• Who killed him? – A burglar. (A burglar killed him).
• Who ate the last piece of cake? – He did. (He ate the last cake.)
• Who wants to go to the cinema? – I do. (I want to go to the cinema.)
• What kept you so late? – My work. (My work kept me late.)
• What happened to my father? – Nothing. (Nothing happened to your father.)
“When did (it) happen?”
This is an object question because “when” is NOT the subject of the verb happened. In a strange way, this kind of makes “when” as the object while “it” is the subject of the verb.
For example, “What did he write?” – This is an object question. Although “he” is the subject of the question itself, the object of the verb write is actually “an essay” – in the answer (represented by “what”). When a question is about the object, we use an auxiliary verb and an infinitive.
What did he write? – He wrote an essay.
Below are object questions. The verb and its subject are bolded and italicizied. Objects are underlined. We use the normal interrogative structure with two verbs (infinitive + auxiliary):
• What did he do?
• Where does he live?
• When will we arrive?
• Where did it happen?
• Why did it happen?
• When did it happen?
• How did it happen?