Who knocks at the door?
Who does knock at the door?
I am confused here. Usually, first sentence is acceptable. But I did not read about the 2nd one. Is it correct? Is it an emphatic sentence of present indefinite tense?
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Who knocks on/at the door?
Who does knock on/at the door?
The first sentence is grammatically correct, whereas the second is incorrect.
When you start a sentence with the pronoun "who" as the subject in a sentence, you don't need to use the auxiliary "do/does/did". On the other hand, when you use who as the object at the start of a sentence, you need to put this auxiliary after it in front of the subject. For example:
Who do you want to talk to?
who did you meet?