Agreement of the relative pronoun and its antecedent
As the relative pronoun refers back to a noun or pronoun (called its antecedent), it must be of the same number and person as its antecedent. Note that the verb shows the number and person of the relative pronoun. For example, a relative pronoun referring back to a singular noun or pronoun must be followed by a singular verb.
Study the following sentences.
The boy who was dishonest was punished. (Here the relative pronoun who refers to the singular noun boy. It is therefore followed by a singular verb.)
The boys who were lazy didn’t win. (Here the relative pronoun who refers to the plural noun boys. It is therefore followed by a plural verb.)
I, who am your king, will lead you. (NOT I, who is your king, will lead you.)
You who are mighty should be kind. (NOT You who is mighty should be kind.)
They who live in glass houses must not throw stones. (NOT They who lives in glass houses must not throw stones.)
Position of the relative pronoun
To avoid ambiguity, the relative pronoun should be placed as near as possible to its antecedent.
The boy who won the gold medal is the son of my neighbor, Mr. Peter. (= The boy won the gold medal.)
The boy is the son of my neighbor Mr. Peter who won the gold medal. (= Mr. Peter won the gold medal.)