that→在
定语从句中做主语或宾语→用来指人或指物（可以用who, whom, which替换）
※ 必须用that的9种情况：
⑴
先行词是all, everything, nothing, few, little, much , something , anything , none等
不定代词时，必须用that引导。
① 我会告诉你我知道的一切。I shall tell you everything that I know.
② 有什么我能为你效劳的吗？ Is there anything that I can do for you ?
⑵ 当先行词被
序数词或形容词最高级修饰时必须用that引导。
① 我不会忘记我学过的第一课。I can’t ∕ won’t forget the first lesson that I learned.
② 他是我见过的最可恨的人。He is the most hateful person that I’ve seen.
⑶当先行词被all, every, little, few, no等修饰时，必须用that引导。
① 有常识的人都不会相信的。No man that has common sense can believe it .
② 我喜欢你给我的所有的书。I like all the books that you gave me.
⑷当先行词被the only（唯一的）；the very（恰好，正好）；the same（相同的）；the last(最后的)等修饰时，必须
用that引导。
① 你是我唯一喜欢的人。You’re the only person that I like.
② 这就是我正想买的那本书。This is the very book that I want to buy .
⑸当先行词既有人又有物时，必须用that引导。
① 我不知道他们正在谈论的人和事。 I don’t know those persons and things that they are talking about.
② 火车朝正在过铁轨的那个男孩和他的狗驶去。
The train ran over to the boy and his dog that were just crossing the track .
⑹当that用做
关系副词，修饰表示时间的名词时，可以代替when来引导。
① 我不会忘记我第一次
见到你的那一天。I can’t forget the day that ∕ when I first met you.
② 事情发生在我出生的那天。 It happened on the day that / when I was born .
⑺当句子中已有who或which出现时，必须用that引导。
① 正在唱歌的
那个女孩是谁？ Who is the girl that is singing?
② 哪个是他昨晚待过的酒店？Which is the hotel that he stayed at last night ?
（8）当先行词是“to be ”后面的
表语，或者
关系代词本身在从句中作表语时
① The school is quite different from the one that it used to be .
② He isn’t the man that he was .
（9）当主句以here , there 开头时
① Here is the hotel that you’ve been looking for .
② There is a seat in the corner that isn’t taken .