定语从句又称关系子句，放在名词后面，用来修饰说明该名词。形式上由关系代名词（who, which, that）来引导：
I met a friend [who helped me a lot]. à who代替前面人物名词作关系子句的主词
This is the book [which interests me]. à which代替前面的无生名词作关系子句主词
This is the car [that I bought ___ yesterday]. àthat代替the car作关系子句受词
修饰人物名词要用who；无生名词则用 which或that，二者可互换：
This is the car which/that is most popular. à which或that代替主词时不可省略
This is the car（which/that）I bought yesterday. à which或that代替受词时可省略
功能上，定语从句分为限定与非限定从句两种用法。
限定从句是用来限定说明先行名词的属性, 意即在所有可能人选中，选择限定一人一物来修饰说明：
I met a friend (out of all friends) who helped me a lot.
This is the book (out of all books) which I bought yesterday.
非限定从句是用来作同位修饰，只此一人一物，没有别的可能人选，因此无需限定，形式上要用逗号为标记：
I love my father, who helped me a lot. à There is only one father!
关系代名词which还可用来指代前面提到的整件事（句子），形式上也用逗号分开：
I bought a new car, which costs me a fortune. à which指“买新车”这件事
He found a new job, which is indeed a good news. à which指“找到新工作”
以上内容由刘美君教授亲自答疑
