Normally you say:
- they do
- they does.
But normally they is plural, not singular. What happens if they is singular? Can you use say "they does" like you'd say "it does"?
When common nouns are used in Subjects, the verb usually agrees with the noun. So if the noun is singular, we see 3rd person singular agreement. If it is plural we see plural agreement:
However, when a pronoun is used as a Subject, the verb always agrees with the pronoun. It doesn't matter what the meaning of the pronoun is!!
So when we use the pronoun one, it doesn't matter if it means "we" or "you" or "people", the verb is always 3rd person singular.
When the queen uses the pronoun we but she means "I", she still uses plural agreement:
In the same way, when we use they as Subject, we always see plural verb agreement. It doesn't matter if we mean "he" or "she" or "that person" or "those people". The verb agrees with the pronoun, not what the pronoun means:
The Original Poster's question
We need to use do with they. We can't use does:
"They" may be singular, but the verb is spelled as for plural.
Example from enter link description here
When I tell someone a joke, he laughs.
When I tell somebody a joke, they laugh.
You are confusing the difference between singular and plural in this case of the subject and how it is handled when conjugating the verb. Every conjugation table shows the different forms for 1st, 2nd and 3rd person, singular and plural.
And in a conjugation table it will show:
Here, as the present tense, there is no occasion when the conjugation differs. If you were to find something that is not as described in a conjugation table then it would be an indicator of either poor grammar or that the case is not nominative, e.g. if you saw "... I were ..." instead of "... I was ...".