Let's begin by reviewing basic subject-verb agreement. In present tenses, the subject and verb must agree in number. This means that if the subject is plural, the verb must be plural, too. Likewise, if the subject is singular, the verb must be singular. To make a present-tense verb singular, we add s or es. Thus, walks is the singular form of walk, and goes is the singular of go.
Remember also that the usual sentence structure is subject + verb. Questions, however, are arranged a little differently. Let's look at an example of a question first to identify its parts:
Have you seen the movie?
Where are the subject and verb in this question? The subject is you, and the verb is have (helping verb) plus seen (main verb). But the helping verb have comes before the subject. That order doesn't follow the subject + verb structure at all, but it is a very common structure for questions: helping verb + subject + main verb.
The entire verb may even come before the subject. This happens when the verb is a simple form of be (e.g., is, are). Then the structure is verb + subject, as in the example below:
Are you ready to go?
[verb are + subject you]
This reversal of the usual subject-verb order is called inversion. Despite the fact that questions follow an inverted order, the subject and verb must still agree. Let's take a look at an example:
Does she play basketball?
Now, let's break down the example:
What is the subject? she
Is it singular or plural? singular
What is the verb? does (helping verb) + play (main verb)
Is it singular or plural? singular (do + es = does)
Let's look at another example:
Do the team members agree?
What is the subject? team members
Is it singular or plural? plural
What is the verb? do (helping verb) + agree (main verb)
Is it singular or plural? plural (do doesn't end in s)
For more information on subject-verb agreement, read Basic subject-verb agreement.