当先行词是everything, anything, nothing （something 除外）, all, none, few, little, some等不定代词时，或当先行词受every, any, all, some, no, little, few, much等词修饰时。如：
Have you set down everything that Mr Li said?
There seems to be nothing that is impossible to him in the world.
注意： 当先行词指人时，偶尔也可用关系代词who。如：
Any man that / who has a sense of duty won’t do such a thing.
All the guests that / who were invited to her wedding were important people.
2. 当先行词被序数词修饰时。如：
The first American movie that I watched was the Titanic.
3. 当先行词被形容词最高级修饰时。如：
This is the best museum that I have visited all my life.
4. 当先行词被the very, the only, the first / last等修饰时。如：
She is the only person that understands me.
After the big fire, the old car is the only thing that he owns.
注意： 当先行词指人时，偶尔也可以用关系代词who。如：
Wang Hua is the only person in our school who will attend the meeting.
5. 当先行词前面有who, which等疑问代词时。如：
Who is the man that is standing in front of the crowd?
Which is the room that Mr Wang lives in?
6. 当先行词为人与动物或人与物时。如：
Look at the man and his donkey that are walking up the street.
7. 当先行词是reason, way（方法）等词时，关系代词常用that代替in which, for which, why, 也常可省略。如：
She admired the way （that） they solved the questions
非限制性定语从句就是用逗号隔开的句子，只能用which