定語從句是英語學習的一個重要語法項目，也是英語考試的熱點之一。小編下面主要跟大家分享下定語從句的引導詞，供大家參考。
1. 根據引導詞在定語從句中的語法 功能進行選擇。
① I will never forget the days when I stayed in Peking University.
② I will never forget the days which / that I spent in Peking University.
③ He is no longer the boy that he used to be.
句①中 stayed 是不及物動詞，引導詞 when 在從句中作時間狀語。
句②中 spent 是及物動詞，引導詞 which / that 在從句中作 spent 的賓語。
句③中引導詞 that 在從句中作表語。
2. 根據先行詞及先行詞前的修飾詞來選擇。
① This is the most beautiful place that I visited last year.
② This is the place which / that I visited last year.
句①中先行詞 place 前有形容詞最高級修飾，引導詞只能用 that。
句②中先行詞 place 前沒有形容詞最高級修飾，引導詞用 which 或 that 均可。
要注意，在下列情況下只能用 that 而不能用 which。
(1)先行詞前有序數詞或形容詞最高級修飾時。
例句：It's the first composition that I have written in English.
(2)先行詞前有 any, all, few, little, the only, the right, the very, the last 等修飾時。
例句：You can take any seat that is free.
(3)先行詞是不定代詞 all, some, something, anything, nothing 等時。
例句：Is there anything that I can do for you?
( 4 )先行詞中既有人又有物時。
例句：He told us the people and places that he had visited.
( 5 )在 there be 結構中，先行詞指物時。
例句：There is a book on the desk that I am interested in.
( 6 )在由 which, who, whom 等開頭的疑問句中。
例句：Which is the book that interests you most?
3. 根據引導詞在定語從句中的位置及意義進行選擇。
① Bamboo is hollow, which makes it very light.
② as is known to all, is a part of China.
③ As anybody can see, the elephant's nose is like a snake.
關係代詞as和which在非限制性定語從句中均可指代主句的全部或部分內容。但 which 引導的非限制性定語從句只能放在句末，如句①，而as引導的非限制性定語從句既可放在句末，也可放在句中或句首，如句②和句③。另外，which引導的非限制性定語從句常與主句有種因果關係，可譯作“所以”，語氣不強時可不必譯出。as引導的非限制性定語從句修飾整個主句時，往往與系動詞搭配使用，可譯作“這一點，正如”。
4. 根據定語從句的性質進行選擇。
① The park, which we visited yesterday, is very beautiful.
② The park which / that we visited yesterday is very beautiful.
非限制性定語從句不能用 that 來引導，並且作賓語的`引導詞不能省略;而限制性定語從句不受限制。句①就是非限制性定語從句，故引導詞只能用which，句②是限制性定語從句，故引導詞用which或that均可。
5. 根據定語從句中介詞所在的位置進行選擇。
① This is the room in which I used to live.
② This is the room which / that I used to live in.
定語從句中如介詞前置，作其賓語的引導詞就不能用 that ，而只能用 which ，且不可省略，故句①只能用 which ;句②則用 which 或 that 均可，且可省略。但要注意，含有介詞的短語動詞一般不能將介詞拆開前置，只能放在動詞之後。
例句：The babies whom the nurses are looking after are very happy.
6. 關係代詞 as 引導定語從句時，常與 the same, such, so 等搭配。
例句：He is doing the same work as I am.他正在做着和我一樣的工作。
7. that 有時被用作關係副詞，代替 in / on / for which ，並且常常被省略。
例句：The way ( that / in which ) you look at problem is wrong.
