★ 先行词前面有最高级形容词修饰时
He is the most boring speaker that I have ever heard.
This is the most interesting book that I have ever read.
★ 先行词有以下代词或者有下列代词修饰时
the first
the last
the only
the same +先行词 +that
the very
all
any
no
every
E.g. This is the last chance that you have.
This is the very thing that I lost yesterday.
This is the first thing that he wants to do.
You are the only friend that we have.
He is the last person that Tom wants to see.
He has told the police all that he knew.
★ 前面已经有疑问词who或者which时
E.g. Who that knows him will trust him?
Which of these trains is the one that goes to Nanjing?
Which is the bike that you have lost?
★ 兼指人和物/事物时
E.g. The man and the horse that fell into the river were drowned.
Look at the girl and her dog that are crossing the bridge.
在下列情况中则多用which。如：
★ 离先行词较远时
He told me the story of the craftsman which I narrated at the beginning of this book.
★ those + 复数名词 之后
You should buy one of those clothes which sell best.
★ which比较正式，非正式英语中也可以用that
Some water experts are now demanding that existing infrastructure be used in smarter ways rather than building new facilities, which is increasingly considered the option of last, not first, resort.
★ 描述性定语从句一般用which
Beijing, which was China's capital foe more than 800 years, is rich in cultural and historic relics.
as, than, but也可以用作关系代词。如：
★ The project was completed on time, as was expected before.(关系代词as在定语从句中用作主语，其先行词是前面的整个句子)
★ They have never met such things as he met.(关系代词as与指示代词连用，在定语从句中用作宾语，其先行词是such things)
★ Tourism, holidaymaking and travel are these days more significant social phenomena than most commentators have considered.(关系代词than在定语从句中用作宾语，其先行词是social phenomena)
