There are many (duplicate) questions about the acceptance, popularity and history of singular they (and their, them and themself) around here, it even got a tag of its own. If I didn’t miss anything, however, the proper verb form hasn’t been questioned yet.
As we all know, English third person singular pronouns (it, she, he and one, +body), names (Alice, Bob, …) and nouns (student, teacher, …) demand the +(e)s suffix be added to the finite verb form in simple present, where some auxiliary verbs have a “special” form (is < *bes / *ares and has < *haves). All other subjects don’t, including plural third person pronoun they.
When the plural you replaced thou (with thee, thy / thine), the other marked verb form that had remained in English – i.e. suffix +(e)st or +t – vanished, too. The first and second persons only ever appear as pronouns (I, we; you), not names or nouns, so there was no strong inclination to keep the verbal inflection.
The second person precedent would suggest that singular they be used with uninflected verb forms which is how it’s usually encountered in the wild. Assuming that they someday replaces he and she (and maybe it) it would lead to disagreement with the words the pronoun stands in for:
- Alice goes to her place by herself.
- Bob goes to his place by himself.
- Alice and Bob go to their place by themselves. – (common)
- Alice and Bob go to their places by themselves. – (separate)
- She goes to her place by herself.
- He goes to his place by himself.
- They go to their place by themselves.
- They go to their places by themselves.
- They ?goes to their place by themself.
- ditto
- They go to their place by themselves.
- They go to their places by themselves.
- They ?go to their place by ?themselves.
- ditto
- They go to their place by themselves.
- They go to their places by themselves.
So why doesn’t singular they afford s on present-tense verbs like all other third person singular subjects do?
I learned about singular they only long after I had been taught English as a second language in school. That’s why it’s still a conscious decision to use it and hence I could easily adapt to use s forms with it, but would that sound and look funny / strange / wrong to native speakers?
It is often said that verbs in English inflect to agree with the person and number of the subject. Now person here cannot be construed as an actual property of the subject. We cannot say for example that the first person is the person who's speaking and the second person is the person being spoken to or any ideas like that. If we do not use an actual pronoun to represent the person speaking, then the verb will not inflect in any way to agree with the 'speaker-hood' of the subject.
- *Araucaria am writing this. (wrong)
- *The reader are reading this. (wrong)
In the sentences above, even though Araucaria refers to the person who is currently speaking to you, in other words me, we cannot use a 'first person' form of the verb. Similarly, even though the reader refers to you, the 'listener', we cannot use a second person form of the verb. When we use common nouns we do not see agreement for person.
Although with common nouns, and proper nouns English verbs inflect to agree with number, they do not agree with any other property of the noun, including the relation to speaker or listener or third party. However, verbs do seem to inflect according to which pronoun is being used. In other words pronouns override the normal agreement of verbs with subjects:
- I am writing this
- You are reading this.
The examples above are fine, not because I refers to the person who is speaking, and you are the person reading, but because verbs inflect in accordance with specific pronouns, and these pronouns override the normal agreement that we see with common nouns.
In the Original Poster's question, this issue is disguised, because when verbs agree with third person singular and plural pronouns, they mimic their behaviour with common nouns. However, this is just an illusion. As with the pronouns I, you and we, 'they' also overrides the normal agreement of verbs with common nouns. Whether used with singular or plural meaning, pronouns always dictate the agreement of the verb according to which actual pronoun they are. They always takes a 'third person plural' form of the verb.
This same phenomenon can be seen with the pronoun one. Whether used to reflect first person, second person, or people in general, one always takes the same verb agreement, the one we wrongly describe as 'third person singular'. The same also applies to royal we.
The answer to the Original Poster's question, therefore, is that verbs won't inflect to reflect the singularity of singular they, because although verbs agree with number when they have common noun subjects, using a pronoun as subject will override the normal common noun agreement and cause the verb to agree according to which specific pronoun is being used. It wouldn't be a good idea to use 'third person singular' forms with they, because it would just be ungrammatical. It wouldn't reflect anything about the meaning of they. The agreement of English verbs with pronouns never reflects any semantic property of the pronoun in the first place!
Will the agreement taken by they change in the future? I don't think so, but I don't know, and I don't know anyone who really does!
[Readers who are interested in this question might also be interested in: Why is "be" the only English verb that inflects for grammatical person, not just for grammatical number like all the rest of them? - although it is a slightly strange question!]