A co-worker of mine took an ELL test for her job and she had to compose a question.
She wrote:
"Does she plays?"
I know that in a statement rather than a question, the third person singular subject is in effect necessitating the 's' on the end of play.
But what is the rule in questions that makes the 's' not needed? I don't know how to explain why we don't do this in a question.
Best Answer
Typically, only the first verb in each clause can be finite:
As you can see, this verb changes form. It shows tense (present or past) and agrees with the subject (adding -s if the subject is third person singular present). This must be our finite verb.
Our example is a declarative clause. To make it into a question, we should turn it into an interrogative clause. And for this sort of sentence, we do that by swapping the subject and auxiliary verb, like this:
This is called Subject-Auxiliary Inversion (SAI). We use SAI to mark these sentences as questions. And in this case, SAI is possible because we have the auxiliary be.
But what if our example doesn't have an auxiliary verb? Let's look at our example from earlier:
We can't swap the subject and auxiliary verb if we don't have one! Let's solve that problem by adding the meaningless ("dummy") auxiliary do:
Now we've got two verbs! And only the first can be finite. In this case, that means do! And as you can see, do now changes form, but play doesn't! That's because play is no longer finite.
Now we can make it into a question. So, let's do Subject-Auxiliary Inversion again, moving the auxiliary before the subject I:
These questions are formed correctly. But what about your example?
It doesn't work! Why not? Because we'd have two finite verbs.
In this answer, the * symbol marks a sentence as ungrammatical, and bold in examples marks a verb as finite.