一、关系代词辨析
1. 关系代词that与which
先行词指物时，that与which一般可以互换，但也有特殊情况。
★通常使用that的情况：
1）先行词为all, everything, something, anything, nothing, much, few, little, none等不定代词或先行词被序数词、形容词最高级或few, the only, little, no, all, every, the very等修饰时。如：
I’ve made up my mind, and nothing that you say will change it.
Jason was probably the hardest working student that I’ve ever taught.
The only thing that matters is that the baby is healthy.
2）当主句是以who, what等开头的特殊疑问句时。如：
Who is the girl that is sitting by the lake?
What is your favorite book that you mentioned just now?
★通常使用which的情况：
当引导非限制性定语从句或关系代词前有介词时。如：
She was fond of music, which her brother never was.
The goals for which he had fought all his life no longer seemed important to him.
2. 关系代词which与as
which和as都可引导非限制性定语从句，用来指代整个主句或主句的一部分。通常which引导的从句放在句末，而as引导的从句既可位于句首，又可位于句中或句末。如：
As is known to the world, Mark Twain is a great American writer.
The size of the audience, as we had expected, was well over one thousand.
He is an American, which / as I know from his accent.
二、“介词 关系代词”引导的定语从句
1. 在“介词 关系代词”引导的定语从句中，关系代词指物时只能用which，指人时只能用whom。如：
The valley in which the town lies is heavily polluted.
We’ll go to see the famous writer about whom we have talked.
2. 在“介词 关系代词”引导的定语从句中，介词的选用取决于句子意义的需要，或句中动词、形容词、副词与介词的搭配。如：
In the office, I never seem to have free time until after 5:30 p.m., by which time many people have gone home.
I couldn’t understand the woman to whom I talked on the phone.
3. “介词 关系代词”前可有名词、数词或some, any, none, both, all, neither, most, each, few等代词。如：
The classroom, the door of which is broken, will soon be repaired.
He loves his parents deeply, both of whom are very kind to him.
4. “介词 which”引导定语从句时，介词可以是in (the) front of, at the foot of, in the middle of等复合介词。如：
He lived in a big house, in front of which stood a big tree.
三、特殊先行词后的定语从句
1. 先行词是reason时，如果关系词在从句中作状语，用why或for which引导；关系词在从句中作主语、宾语或表语，用that或which引导。如：
We’d like to know the reason why / for which she didn’t accept the job.
There is another reason that / which is more important.
2. 先行词是表示方式的way时，定语从句可用that或in which引导，也可省略。如：
People liked the way (that / in which) he wrote, so he went on writing.
3. 先行词是stage, position, situation, atmosphere等有地点含义的抽象名词，且关系词在定语从句中作状语时，用where引导。如：
Their youngest child is at the stage where she can say individual words but not full sentences.
You could get into a situation where you have to decide immediately.
4. 先行词是occasion时，一般要用when引导定语从句。如：
I can remember very few occasions when he had to cancel our trips.
四、关系词的省略
1. 关系代词在从句中作宾语时可省略。如：
The cup (that / which) you bought is broken.
2. 在某些表示时间、地点的词后的限制性定语从句中，关系副词在口语中有时省略。如：
I’ll never forget the year (when) I stayed in Beijing.
This is the place (where) I lived five years ago.
注意：非限制性定语从句通常由who, whom, which, whose, when, where及“介词 which / whom”引导，且关系词一般不省略。另外，在非限制性定语从句中，which可指代整个主句内容。
