Ostensibly, verb conjugation is plural vs singular: "The apple is big." vs "The apples are big."
But that doesn't match up with the centuries-old singular they.
"They is nice." vs "They are nice."
I believe only the latter is correct, not only for plural they but even for singular they.
Are there any other exceptions to the singular vs plural verb conjugation? Or is singular they a unique exception to English verb conjugation among all subjects?
Is plural vs singular still the way that grammarists explain English verb conjugation?