3

She does her homework.

Does she do her homework?

Why the questioning sentence is using "do"?

4

Does she do her homework?

To ask a question, we need to use an auxiliary verb at the beginning of the sentence. In our case, the auxiliary verb is do: this is called do-support.

But since "she" is a singular third-person pronoun, we turn do into does. The first do carries the third-person ending s(or es) instead of the second. So there's no reason to leave the second verb with the same ending.

Compare:

She flew home. ("Flew" is the Past Tense form of "fly")
Did she fly home?

Notice how the auxiliary verb do assumed the Past Tense form did, and the main verb fly now has no need to be in the Past Tense form: the auxiliary verb does this work.

The main verb contains the meaning, the auxiliary verb is a "helping verb".

In your example sentence, the first do (in the form does) is an auxiliary verb, and the second do (in the form do) is a main verb.

  • 1
    Very nicely did (American regionalism) – Tᴚoɯɐuo Nov 30 '14 at 18:42
1

The usual situation

When turning a statement into a question, we move the first word of the verb to the beginning of the sentence:

Statement: She is doing her homework.
Question: Is she doing her homework?

 

An exception

If there is only one word in the verb, we use a form of to do to add a word to the verb, then move the added do:

Statement: She does her homework.
Two-word verb: She does do her homework.
Question: Does she do her homework?

 

Statement: She has a question.
Two-word verb: She does have a question.
Question: Does she have a question?

 

An exception to the exception

If the verb is a one-word form of to be, we can't use to do to add a word. All we can do is move the verb that exists.

Statement: She is a student .
Mistake: She does is a student.
Question: Is she a student?

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