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Questions with what, who and whom
The question word what and who can act both as a subject or an object. Depending on that, the structure of the sentence will be different.
Study these examples
What did you see?
Who owns this company?
Who did Mike call?
Questions to the subject with what and who
If "what" or "who" is the subject in the sentence, we don't need an auxiliary verb. The main verb goes right after the question word:
What/Who + main verb + ...?
Examples of what and who as a subject
What is her name? Her name is Christy.
What happened to Paul? He fell down the ladder.
What is hanging on the wall? A painting is hanging on the wall.
What makes you happy? Love makes me happy.
Who is knocking at the door? A postman is knocking at the door.
Who made this armchair? My grandfather made it himself.
Who knows the answer? Jenny knows the answer.
Questions to the object with what and who
If what/who is the object in the sentence, we need an auxiliary verb.
What did you learn at the lesson? I learnt some new grammar rules.
What does John want from me? He wants some help.
What has the dog been doing? The dog has been swimming in a mud puddle.
Who is Amanda calling now? She is calling a friend of hers.
Who do we believe? We believe them.
Who did they sell their car to? They sold it to the Fishers.
Who and whom
We use "what" both as a subject and an object to make questions about things, abstract terms, but not about people.
We use the question word "who" when we ask questions about a person.
If the question word is a subject (= we can replace it with a personal pronoun in its basic form: Who did it? He did it. Who cares? She cares.), then we always say "who".
But, if the question word is an object (= we can replace it with a personal pronoun in the objective case: Whom do you help? I help him.), we should say "whom" according to the official grammar rules. Especially, when the verb requires a preposition. However, whom is rarely used in modern spoken English, it sounds very formal.
Examples of whom with a preposition
Whom is he talking to? (=Who is he talking to?)
Whom are we going to the theatre with? (= Who are we going to the theatre with?)
Whom did you bake the cake for? (= Who did you bake the cake for?)
If we sum up the above written, we'll get a following table:
question to subject
question to object
what/who + main verb + ...?
what/who + aux. verb + subject + main verb + ...?
What makes you happy? Who has just called?
What did you make at the art lesson? Who have you just called?
also possible whom Whom are you going out with? Whom did they offer the job to?