關係代名詞具有連接詞和代名詞的功能。
它連接主要子句及關係子句(或稱形容詞子句，屬後位修飾法)。
它代替主要子句的主詞或受詞(稱為先行詞)。
先行詞若是人時，關係代名詞當主格用(不可省略)，用who/that
關係代名詞當受格用(可省略)，用whom/who/that
關係代名詞當所有格用(不可省略)，用whose(of which) 後接名詞
先行詞若是事物時，關係代名詞當主格用(不可省略)，用which/that
關係代名詞當受格用(可省略)，用which/that
關係代名詞當所有格用(不可省略)，用whose(of which) 後接名詞
先行詞若是人和事物時，關係代名詞當主格用(不可省略)，用that
關係代名詞當受格用(可省略)，用that
關係代名詞當所有格用(不可省略)，用whose(of which)
關係代名詞當主格用
(1)慧星式(關係子句放句尾，修飾主要子句的受詞。關係代名詞當主格用(不可省略)，先行詞若是人時，用who/that。先行詞若是事物時，用 which/that)
I like the person. The person has a good heart. (用關代合併)
(受詞)
I like the person who/that has a good heart.
(先行詞，人)
└ 主要子句 ┘└ 關係子句 ┘
She likes the doll. The doll is wearing a skirt. (用關代合併)
She likes the doll which/that is wearing a skirt.
(先行詞，事物)
(2)三明治式(關係子句放中間，修飾主要子句的主詞，關係代名詞當主格用(不可省略)，先行詞若是人時，用who/that。先行詞若是事物時，用 which/that)
The boy is my cousin. The boy sings beautifully. (用關代合併)
(主詞)
The boy who/that sings beautifully is my cousin.
(先行詞，人)
└ 關係子句 ┘
└ 主要子句 ┘
The cats are mine. The cats have big eyes. (用關代合併)
The cats which /that have big eyes are mine.
(先行詞，事物)
關係代名詞當所有格用
(1) 慧星式(關係子句放句尾，修飾主要子句的受詞，關係代名詞當所有格用(不可省略)，先行詞若是人時，用whose後接名詞。先行詞若是事物時，用whose後接名詞)
I like the girl. The girl’s eyes are big. (用關代合併)
(受詞)
I like the girl whose eyes are big.
(先行詞，人)
└主要子句┘└ 關係子句 ┘
I like the dog. The eyes of the dog are big. (用關代合併)
I like the dog whose eyes are big.
(先行詞，事物)
(2) 三明治式(關係子句放中間，修飾主要子句的主詞，關係代名詞當所有格用(不可省略)，先行詞若是人時，用whose，後接名詞。先行詞若是事物時，用whose後接名詞)
The cat is mine. The name of the cat is Kitty. (用關代合併)
(主詞)
The cat whose name is Kitty is mine.
(先行詞，事物)
└ 關係子句 ┘
└ 主要子句 ┘
關係代名詞當受格用
(1) 慧星式(關係子句放句尾，修飾主要子句的受詞，關係代名詞當受格用(可省略)，先行詞若是人時，用who/whom/that。先行詞若是事物時，用 which/that)
This is the woman. Mom saw the woman on the street. (用關代合併)
This is the woman (who/ whom/ that) Mom saw on the street.
This is the dog. Mom saw the dog on the street. (用關代合併)
This is the dog (which/that) Mom saw on the street.
(2) 三明治式(關係子句放中間，修飾主要子句的主詞，關係代名詞當受格用(可省略)，先行詞若是人時，用who/ whom /that。先行詞若是事物時，用 which/that)
The woman is a doctor. Tina is talking to the woman. (用關代合併)
The woman (who/ whom/ that) Tina is talking to is a doctor.
The dog is black. Tina bought the dog yesterday. (用關代合併)
The dog (which/that) Tina bought yesterday is black.
