Many people have problems with relative pronouns. who and who For one simple reason: when they were young in school, they never learned the difference between a nominative and an objective case.
The subject of a verb is in the nominative case. The nominative forms are I, we, he, she, they and who.
Correct: I forgot who won the game. (Who is the subject of the clause).
Correct: John and me joined a sports team at school. (John and I are composite subjects).
Right: when Michael comes, he and I we are going to build a boat. (He and I are subject to clause).
Correct: U.S girls can do a better job without boys. (We are the subject).
Correct: it was Hey. (The pronoun follows a linking verb and functions as a nominative predicate.)
Correct: Class officers are Marianne, Robert and I. (Marianne, Robert and I are nominative predicates).
The object of a verb or a preposition is in the objective case. The objective forms are me, us, him, her, them and who.
Correct: I don’t know who voted for. (Who is the object of the preposition).
Correct: Father puts Olga and I at the bus station. (Olga and I are objects of the verb met.)
Correct: All the plans for the party were made by Fred and her. (Fred and she are objects of the preposition of.)
Correct: Enter John and the there has always been a true friendship. (John and he are objects of the preposition between).
Note: The indirect object is also in the objective case. The indirect object is the object of for or to be understood.
Correct: Aunt Mary smells I a necklace from Bangkok. (The necklace is the direct object; I is the indirect object.)
Correct: Save me a piece of cake. (I am the object of course; the piece is the direct object).
Problem areas. Many speakers and writers think that it is more elegant to say John and I than John and I. However, that is not necessarily true. John and I are right when used in the nominative case, and John and I are correct when used in the objective case.
Wrong: the teacher gave her full attention to John and me. (It should be John and me).
Correct: John and me will participate in the 10 km race.
Another common mistake in speech and writing occurs in relation to the pronoun that is linked to a noun in the objective case, as with us students or us men.
Correct: various from us girls decided to go home for Thanksgiving.
To break the rules. In informal conversation, expressions like Who did you call? they are common, and most educated people accept them as appropriate in informal situations, though not in formal writing.