什么是先行词,在什么时候要用that,什么时候用which什么时候用who...........
楼上对先行词的理解基本正确,但举例中第三句没这样的表达,可以改为:I saw the woman who was Mrs.**.
最后两句是宾语从句的例子,句中也就没有先行词可言了.
何谓定语从句？顾名思义，作定语的句子就叫作定语从句。定语从句所修饰、限定的词叫先行词。定语从句通常跟在和它发生关系的先行词之后，它与先行词有着不可分割的联系。它通常是由关系代词(who，whom， whose，that，which，as)和关系副词(where，when，why， how)来引导。关系代词、关系副词在从句中需担当成分，但无实际意义。初中阶段我们只要理解who，that，which这几个关系代词所引导的定语从句即可。
1.关系代词who。关系代词who在从句中作主语，它的先行词是指人的名词或代词。例如：
(1)Find the boy who is wearing a red shirt.找出穿红衬衫的那个男孩。(先行词是 the boy)
(2)He who does not reach the Great Wall is not a true man.不到长城非好汉。(先行词是He)
2.关系代词that。关系代词that在从句中可作主语、宾语、表语。作宾语时，that可省略。它的先行词可以是指物的名词或代词，也可以是指人的名词或代词。例如：
(1)She told her mother all that had happened.她把所发生的事情都告诉了她的母亲。(先行词是all，that在从句中作主语)
(2)I'll buy you the bike(that)you saw in the shop yesterday.我要给你买昨天你在商店里看到的那辆自行车。(先行词是the bike， that在从句中作宾语)
(3)The village is no longer the village that it was before.这个村子不再是从前的那个村子了。(先行词是the village，that在从句中作表语)
(4)He that wants to eat the fruit must climb the tree.想吃果子的就得爬树。(先行词是指人的代词he，that在从句中作主语)
3.关系代词which。关系代词 which在从句中作主语或宾语，其先行词是指物的名词或代词。例如：
(1)They first went to a large room which looked like a shop.他们首先到一个很大的看起来像个商店的房间去了。(先行词是指物的名词a large room， which在句中作主语)
(2)Have you got anything which you'd like to buy？你有想要买的东西吗？(先行词是指物的代词anything，which在从句中作宾语)