Who and `do support'

Q: Masoumeh A. asks,

"I have some grammar questions which look very simple but somehow they make me confused… someone told me that we can't use do, does, did after who, for example:

1.
a) Mary helped me to come to Canada.
b) Who helped you to come to Canada?
c) Who did help you to come to Canada?"

A: Unfortunately, it seems that Masoumeh's informant has only explained part of the rule. A more complete version is:

An auxiliary verb is needed in open questions, unless the question is about the subject. When the question is not about the subject and there is no auxiliary verb, then we add do.

The details and the missing piece follow:

What we're looking at here is something called do support or do insertion. The fact is that in most open questions (questions using a question word: who, what, how, etc.) we need some kind of auxiliary verb, and when there isn't one, we recruit do to do the job for us. Consider these examples:

2.
a) Where would you get a forty-foot sword sharpened, anyway?
b) *Where ___ you get a forty-foot sword sharpened, anyway?
c) Where do you get a forty-foot sword sharpened, anyway?

In 2a, the auxiliary verb is would. In 2b, there is no auxiliary verb, and the sentence ends up being ungrammatical, so we insert do, resulting in the grammatical 2c version. This is always true with open questions formed with when, where, why, and how, which are not pronouns and therefore cannot function as subjects: these sentences always need an auxiliary verb. But it is only sometimes true with who, what, and which, and it depends on whether or not the question word functions as the subject of the sentence (or part of the subject, see example 5 below).

If the question word replaces an NP (or part thereof) that is not a subject, then usually the NP becomes who, what, or which. Next, if there's no auxiliary verb, do is added in front of the main verb. Third, the subject and the auxiliary verb switch places, and fourth, the question word is fronted as in examples 3 and 4.

3.
a) They like pie.
b) They like what? (change the noun phrase (NP) to a question word)
c) They do like what? (there's no auxiliary verb, so add do before the main verb)
d) do they like what? (switch the subject and the auxiliary)
e) What do they like? (front the question word)

4.

a) They like her.

b) They like who? (change the noun phrase (NP) to a question word)

c) They do like who? (there's no auxiliary verb, so add do before the main verb)

d) do they like who? (switch the subject and the auxiliary)

e) Who do they like? (front the question word)

Sometimes, though we not just asking what in general (e.g., what was it?). We know the noun, but we don't know its specific referent: it's pie, but is it this pie, or that? Here, we can use whose or the determinatives what and which by themselves (e.g., which?) or together with the noun (e.g., which pie?). Either way, this NP with the question word moves to the front as in 5c and do is added.

5.

a) They like this pie.

b) They like what/which/whose pie? (change the determiner¹ to a question word)

c) They do like what/which/whose pie? (there's no auxiliary verb, so add do before the main verb)

d) do they like what/which/whose pie? (switch the subject and the auxiliary)

e) What/which/whose pie do they like? (front the question word)

This all applies when the question word does not function as the subject or its determiner. Asking about subjects is much easier, since the subject is already at the front of the clause. In this case, there is only one step: change the subject (or its specifier) to the question word:

6.
a) She likes pie.
b) Who likes pie? (change the NP to a question word)

7.
a) This works.
b) What works? (change the NP to a question word)

8.
a) This machine works.
b) What/which machine works? (change the specifier to a question word)

Since there is no subject-auxiliary inversion, there's no requirement for do insertion. So, the rule isn't that you can't use do with who, but rather that:

when the question word is the subject or its determiner, do insertion isn't needed.

That's not the whole story though. Notice that, unlike 2b, 1c isn't starred. In other words, 1c is grammatical. Even though we don't need do insertion, do can be used when there is no auxiliary verb to add emphasis, as in the following conversation:

A: Somebody helped me come to Canada.
B: Was it a lawyer?
A: No, I didn't have a lawyer.
B: Was it your parents?
A: No, they tried to stop me.
B: Then who did help you come to Canada?

In sum, then, an auxiliary verb is needed in open questions, unless the question word functions as the subject or its determiner. When it is not, and when there is no auxiliary verb, then we add do. We can also use do insertion for emphasis when the sentence has no auxiliary verb.

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[1] Determiner is a function in noun phrases. It is usually performed by words belonging to the category  of determinatives such as this, many, a, and which. Possessive NPs such as Juan's, the dog's, my, or whose also commonly function as specifiers. Traditional grammar would simply call this an adjective functioning as a modifier.