1. Reduced Relative clauses (Who, whom, which)
You may delete the relative pronoun and the be verb when:
they are followed by a prepositional phrase.
A. The man who is in the house is my father.
The man in the house is my father.
B. The books that are on the desk are mine
The books on the desk are mine.
2. the main verb in the relative clause is progressive.
A. The man who is swimming in the lake is my father.
The man swimming in the lake is my father.
B. The books that are lying on the floor are mine.
The books lying on the floor are mine

2. Reduced Relative clauses (Who, whom, which)
You may NOT delete the relative pronoun and the be verb when :
1. they are followed by an adjective:
The man who is angry is my father
X The man angry is my father
(However, you may switch the positions of the adjective and noun.)
The angry man is my father. (okay)
2. they are followed by a noun:
The man who is a doctor is my father.
X The man a doctor is my father.

3. Reduced Relative clauses (Who, whom, which)
The relative pronoun can be deleted if there is a new subject and verb following it:
A. This is the house that Jack built.
B. This is the house Jack built.
A. The person whom you see is my father.
B. The person you see is my father.

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