一、用于引导名词性从句的引导词有以下几类：
- 连接代词： what, which, who, whom, whose
- 连接副词： when, where, why, how
- 连接连词： that （宾语从句或表语从句中"that"有时可以省略）；whether,if （均表示“是否”表明从句内容的不确定性）；as if ,as though （均表示“好像”，“似乎”）
二、what, which在从句中作主语、宾语、表语或定语；who在从句中作主语、表语；whom在从句中作宾语；whose在从句中作定语；when, where, why, how在从句中分别作时间状语、地点状语、原因状语、方式状语。例如
1.choose which you like best.
选你最喜欢的。
2.who has taken away my bag is unknown.
谁拿走了我的包还不知道。
3.have you decided whom you are to nominate as your candidate?
你们是否已经决定提名谁做候选人了?
4.he asked when mr. gilbert would be allowed to go home.
他问什么时候可以允许吉尔伯特先生回家。
5.where we can look up his address is still a problem.
我们在哪儿可以查到他的地址还是个难题。
6.why he did that wasn’t quite clear.
他为什么做那件事还不十分清楚。
7.how they will solve the serious problem has not been decided.
他们将怎样解决这个严重问题还没有决定。
三、what作连接代词引导的名词性从句既可以表示一个问“什么……？”的问题， 也可以表示相当于“名词/代词＋关系代词引导的定语从句”结构的意思， 这种特殊用法在语法上被称作“关系代词型的what”，例如：
1. he is not what (=the person that) he was a few years ago.
他不是几年前的他了。
2. this is what (=the place that) they call salt lake city.
这就是他们称作盐湖城的地方。
3. what (=the place that) is now the north sahara desert was once a civilized world.
现在的北部撒哈拉沙漠曾是一个文明世界。
4.our income is now double what (=the income that) it was ten years ago.
我们现在的收入是十年前的两倍。
5.the color of the flower is different from what (=the color that) it was in the morning.
花朵(现在的)颜色与它早上的颜色不同。
6.how did the cheats explain the fact that the emperor didn't feel the new clothes in him?
皇帝觉得身上并没有穿上新衣，对这个问题骗子是怎样解释的?
7.i owe it to you that i am still alive.
幸亏你，我现在还活着。
8.she made a request that the doctor(should)be sent for at once.
她请求立即请医生来。
9.you must see to it that the children don't catch cold.(=you must see to it,and it is that the children don't catch cold.）
你要当心，别让孩子们着凉了。
10.she told me that she would accept my invitation.
她告诉我，她接受我的邀请。