大家都知道在定語從句中，有的時候where，why，when可以換成「介詞+which」結構，可是很多同學不明白這個結構裡的介詞是怎麼來的。今天我們一起挖一挖，這個介詞的由來。
首先我們要搞清楚，這個介詞的原來位置不是在which前面，它原本是放在從句中的。它應該和從句中的動詞形成一個短語、固定搭配或習慣用法等。
例1：Lili's dream is to have her own garden ______ to grow plenty of flowers.
A. that B. by which C. which D. in which
解析：答案選的是D。這個從句是省略結構。從句和先行詞garden連起來：to grow plenty of flowersin her own garden.把從句和先行詞連起來需要一個介詞in。而介詞in可以提前到which前面，就形成了in which結構。
例2：While driving through the city, she showed me the building ______ she once worked as a lift operator.
A. when B. which C. in which D. by which
解析：答案選C。先行詞是building，把從句和先行詞連起來。She once workedin the building as a lift operator.所以，就有了in which。
例3：The man ___________ you talked is my English teacher.
A. to whom B. with who C. with whom D. that
解析：答案選B。把從句和先行詞連起來：You talked withthe man. 而且the man是介詞with的賓語，所以用賓格whom。
例4：Eric received training in computer for one year, _____ he found a job in a big company.
A. after this B. after that C. after it D. after which
解析：答案選D。which代指前面一整句話。把從句變為主句是這樣的：He found a job in a big company afterEric received training in computer for one year.所以就有了after which。
大家明白了嗎？介詞其實很好確定，只要把從句和先行詞連一下，就很容易看出需要的介詞了。希望可以幫到大家。
如有問題請留言。