The relative pronounA part of speech that refers to or stands instead of a noun. E.g.: Who do you think I am? ("you", "I") can be omitted in defining relative clauseA group of related words containing a subject and a verb.s if the pronoun is the objectA person or thing that is affected by the action of a verb or involved in the result of an action. of the clause. We use the phrase zero pronoun to indicate such a case. There are, however, other cases when the clause can be shortened.

John was the only one who was not informed about the room change.
Who is the girl who sits next to you in class?

In both sentences the relative pronoun who is the subjectThe person or thing that performs the action of a verb or that is described by a verb. E.g.: Andy has never been to Brunei. ("Andy") of the clause, but in the first sentence it is followed by a passive form, while in the second by an active one. We can omit the pronoun if we make some changes: 

John was the only one not informed about the room change.
Who is the girl sitting next to you in class?

The passive verbA part of speech that expresses an action or a state. E.g.: John seldom plays tennis. has been changed into a past participle (third form of the verb), while the active one into a present participle.

Infinitives replacing a relative clause
Non-finite clauses replacing a relative clause Revision questions: relative clauses