"Who appointed him mayor?" Who is an unknown, so would the pronoun number be unknown?
Part of my question entails sub-verb agreement. If verbs are classified as being singular or plural, can they be unknown, hence because the subject who is unknown then would the verb appointed be classified as with the number as unknown?
I am looking for a specific grammar rule or concept for this question. I have looked at many old grammar books on Google Books to no avail.