'do' as an auxiliary verb

Question

Hiya

I'm a native speaker, teaching English online. A student stumped me with a question about using 'do' as an auxiliary verb.

I'd appreciate it much if someone please tell me of a general principle for using the auxiliary verb 'do' that explains why we say, 'What did you do yesterday?' but not 'Who did buy it for you?'. Does it have something to do with the object in the second sentence?

I have referred to a grammar book and an online resource, but neither helped me to understand this. I have been teaching for quite a few years, but never studied grammar, and realize there's quite a lot that I don't know.

Persian Learner · Accepted Answer

anonymousWhat did you do yesterday?

In the above, 'what' is the object of the sentence, hence the use of the auxiliary 'did' for the main verb 'do'.

anonymousWho did buy it for you?

In this sentence, 'who' is the subject of the sentence; thus, the following verb acts as the finite (main) verb. So it would be wrong to use two consecutive finite verbs in different tenses for one subject, (did, past; buy, present).

Who did it?

Who bought it?

And in the following, 'who/whom' functions as the object of the preposition 'for', and the subject is 'you'.

Whom/who did you buy it for?

AlpheccaStars · Answer

anonymous'What did you do yesterday?'

English language has four moods: declarative, (indicative), interrogative, and subjunctive.

To make a declarative sentence ( statement) into an interrogative (question), we invert the inflected verb and the subject.

e.g.

John is cold. -> Is John cold?
John is going to school. -> Is John going to school?
John has bought a new car. -> Has John bought a new car?
Kathy can help you. -> Can Kathy help you?

When there is only one inflected verb, without "be" or "have" or a modal as an auxiliary, we use the auxiliary "do"

Mary helps her mother, -> Does Mary help her mother?
Mary helped her mother. -> Did Mary help her mother?
John has a new car. -> Does John have a new car?

Interrogative pronouns also require the inversion.

What does Mary have?
Why did did Mary help her mother?
Where is John going?
When did John buy a new car?
Mary did her homework. -> What did Mary do?

So a primary use of "do" is to make the interrogative mood.

Another use of "do" is emphatic. The sentence is still a statement.
Using "do" as a regular auxiliary expresses shock or surprise or emphatic agreement.

A: Does John really buy a new car every six months?
B: Yes he does do that.

Here is something on grammatical moods:

https://www.thoughtco.com/mood-in-grammar-1691405
https://www.thoughtco.com/know-your-auxiliary-verbs-1210731

Do is also an auxiliary for negative sentences:

John did not go to school today.
Do your homework!
Don't do that!

anonymousWho did buy it for you?

That is grammatically correct, but unusual. It is the emphatic use of the auxiliary "do."

Who is an interrogative pronoun that acts as the subject of the verb, but we do not use inversion.

Who is going with you?
Who bought it for you?
Who did it?

However, in rare cases, we use "do" as an emphatic auxiliary.

Who killed Roger Rabbit?
Who did kill Roger Rabbit? (emphatic)

Mom: Who went with you?
Jane: Nobody.
Mom: Kate saw you with someone.
Kate: There was nobody.
Mom: There was someone with you! Who did go with you? Who did buy that ring for you?

https://www.thoughtco.com/what-is-the-emphatic-do-1690590

BillJ · Answer

anonymousI'd appreciate it much if someone please tell me of a general principle for using the auxiliary verb 'do' that explains why we say, 'What did you do yesterday?' but not 'Who did buy it for you?'. Does it have something to do with the object in the second sentence?

[1] What did you do yesterday?

[2] Who did buy it for you?

[3] Who bought it for you?

In example like these, it all depends on the function of the question word.

In [1] the fronted question word "what" is object of "do", and the fronting is accompanied by obligatory subject-auxiliary inversion. This type of inversion requires the presence of an auxiliary verb, but in the basic order version You did x yesterday, there is no auxiliary present so the dummy auxiliary "do" has to be inserted.

In [2] the fronted question word "who" is subject and there is no inversion. "Did" is here an optional auxiliary used simply to emphasise the positive polarity of the clause. It has no bearing on inversion, as can be seen in the non-emphatic version [3].

'do' as an auxiliary verb

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