来看几个例句。
Is he the man who wants to see you?
这个句子中的who wants to see you就是定语从句。who是关系代词，在从句中作主语。
She is the woman whom I saw yesterday.
关系代词whom在定语从句whom I saw yesterday 中作宾语，因为正常语序是I saw +人(whom) yesterday，定语从句里把whom提前，作为关系代词引导整个从句。
你可以把定语从句看成是一个以句子形式出现的定语，在第一个句子中定语修饰的是the man，定语（从句）是对the man进行解释说明：那个想见你的（who wants to see you）男人(the man)，翻译成中文时把英语中的定语部分放到“男人”的前面，这样才符合汉语表达习惯。
关系代词除了who,whom以外，还有whose,which,that.
Please pass me the book whose cover is green.请递给我那本绿色封面的书。
The package which/that you are carrying is about to come unwrapped.你拿的包快散了。
除了关系代词可以引导定语从句外，关系副词也可以引导定语从句。关系副词有when,where,why.
Nanjing is the place where I was born.南京是我的出生地。
关系副词where=in which,所以这句话也可以这样说：Nanjing is the place in which I was born.
短语Be born in + 地方，要把born 后面的介词in提前放到which的前面。
什么时候用关系代词，什么时候用关系副词呢？
当从句中的及物动词后面没有宾语时，要用关系代词引导从句。
This is the city which I visited last year.
及物动词visit后面没有宾语，要用关系代词which引导定语从句。
当先行词在从句中做主语，定语，宾语时，要用关系代词引导从句；当先行词在从句中做状语时，要用关系副词引导从句。
Is this the village which you visited a few days ago?
You visited the village a few days ago,所以先行词village在从句中是做宾语,要用关系代词which引导从句。
Is this the museum where the exhibition was held?
The exhibition was held in the museum,所以先行词in the museum在从句中是做地点状语（表示地点），要用关系副词where引导从句（where=in which)。
关系代词who指人，which指物；that用在限定性定语从句中，又可以指人又可以指物。
限定性定语从句不能被省略，如果被省略了，就会影响整个句子的理解；非限定性定语从句起补充说明的作用，即使被省略了，也不会影响全句的理解。