1. 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.
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.
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.
A. This is the place where I live.
B. This is the place I live.
A. I don’t know the reason why she is late.
B. I don’t know the reason she is late
(Also : I don’t know why she is late. )
A. The woman whom he likes is married.
B. The woman he likes is married.
Notes:
Do not delete a relative pronoun that is followed by a verb other than be:
The man who likes lasagna is my father.
X The man likes lasagna is my father.
Never delete the relative pronoun whose:
The man whose car broke down went to the station.
X The man car broke down went to the station.
See also:
Grammar: Relative Clauses
