[Source:] It's also worth noting that the corrected version doesn't use but, but but then. The then is a key word that makes the sentence sensible; it indicates when Joey realized his wallet was at home – after they had reached the head of the line. Without that word, neither but nor yet make apt conjunction.
1. The correlative conjunction 'neither ... nor' is singular, and but and yet here are singular subjects, so should make be makeS?
2. apt conjunction as the predicate looks wrong, but I can't pinpoint why. Should this be an apt conjunction or apt conjunctionS ? Why or why not?