定語從句
定語從句的引導詞有關係代詞who， whom，which，that， as；關係副詞when， where，why；關係形容詞whose。
套用
缺少成份
定語從句中引導詞的選用主要看先行詞在定語從句中作什麼成分或者是定語從句中缺什麼成分。
1．定語從句中缺主語，修飾人時用who /that，修飾物時用which /that引導。如：
Is this the factory which /that makes TV sets？
The man who /that visited our school is from Australia．
2．從句中缺賓語，修飾人時用 who /whom /that或省略引導詞；修飾物時用which /that或省略引導詞。如：
Gone are the days which /that we spent together in the village．
He is the man who /whom /that I think to be worthy of our praise．
3．從句中缺時間狀語，用when或介詞+which引導。如：
The day finally came when /on which I was given a job．
We'll put off the outing until next week，when we won't be so busy．
4．從句中缺地點狀語，用where或介詞+which引導。如：
Please put the letter on the desk where /on which he can easily find it．
5．從句中缺原因狀語或先行詞是 reason時，引導詞用why /for which / that。如：
This is the reason why /for which / that he didn't come to the meeting．
6．從句中缺定語，人和物都用 whose引導。如：
A child whose parents are dead is called an orphan．
Please pass me the dictionary whose cover is black．
7．當先行詞是way時，其定語從句的引導詞用in which /that。如：
The way in which /that he looks at problems is wrong．
as
引導詞as可引導限制性定語從句，也可引導非限制性定語從句。
1．在固定詞組the same．．．as，such．．．as中，as引導限制性定語從句。如：
He is not such a fool as he looks．
I've bought the same dress as she is wearing．
2．當先行詞是整個主句時，可用 as /which引導非限制性定語從句。引導詞as和which的區別在於：
①as引導的非限制性定語從句可放於主句前、主句中、主句後，而which引導的非限制性定語從句一般放於主句後或句中。
②as常與從句中的know，see， hear，expect等動詞連用，也常用於as often happens，as is often the case（常有的事）等句子中；而which一般不用於以上情況。
③as有“正如”的含義，which沒有此含義。如：
As is well known，Shanghai is a big city．
Some of the roads were flooded， which made our journey more difficult．
定語從句
關係代詞who，whom，whose， which和as能引導非限制性定語從句，而that不能；非限制性定語從句中即使缺賓語，引導詞也不能省略；關係副詞when，where能引導非限制性定語從句，而why不能。如：
Abraham Lincoln，who led the United States through the Civil War years，was shot on April 14，1865．
互換規則
先行詞是物時，其引導詞可用 which也可用that，通常情況下二者可互換，但在有些情況下不能。
1．用which不用that引導定語從句的三種情況：
①在引導非限定性定語從句時，和在介詞後不能用that，套用which。如：
Football，which is a very interesting game，is played all over the world．
②先行詞為that時，為了避免重複，定語從句用which不用that引導。
如：That which you borrowed from me wasn't a real diamond necklace．
③介詞後用which不用that引導。如：
The method with which you solved the problem is very good．
2．用that不用which的七種情況：
①先行詞前有形容詞最高級或序數詞修飾時，用 that不用which引導。如：
This is the best place that I have ever visited．
The second question that is asked is why we don't fall off the earth．
②先行詞為little，much，all，none，few，（the）one， something，everything，nothing，anything等不定代詞時，用that引導。如：
There is nothing in the world that can frighten him．
③先行詞既有指人的名詞又有指物的名詞時，用 that引導。如：
He told us about the people and the cities that he had visited．
④先行詞前有any，all，no，few，every，some，little， much，（the）very，（the）only，（the）last等詞修飾時，用 that引導。如：
This is the very coat that I need．
Is oxygen the only gas that helps fire burn？
⑤先行詞為which時，為了避免重複，用that引導。如：
Which is the book that you bought yesterday？
⑥先行詞在從句中作表語時，常用that引導。如：
Shanghai is no longer the city that it used to be in the 1930's．
⑦當主句是There be．．．句型時，用that引導。
There are two tickets for the film that are for you two．
結構
在“one of+複數名詞+定語從句”結構中，從句的謂語動詞應與複數名詞保持一致；當one前有the only修飾時，從句的謂語動詞應與the only one保持一致。如：
He was one of the students who were late for class．
He was the only one of the students who was late for class．
引導詞前有介詞或短語介詞時，修飾人只能用 whom，修飾物只能用which引導，如：
My glasses，without which I was like a blind man， fell to the ground and broke．
The man，by whom the wolf was shot，is called Jackson．
