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.
2.Who has taken away my bag is unknown.
3.Have you decided whom you are to nominate as your candidate?
4.He asked when Mr. Gilbert would be allowed to go home.
5.Where we can look up his address is still a problem.
6.Why he did that wasn’t quite clear.
7.How they will solve the serious problem has not been decided.
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.
2. This is what (=the place that) they call Salt Lake City.
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.
5.The color of the flower is different from what (=the color that) it was in the morning.
6.How did the cheats explain the fact that the Emperor didn't feel the new clothes in him?
7.I owe it to you that I am still alive.
8.She made a request that the doctor(should)be sent for at once.
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.）
10.She told me that she would accept my invitation.