1、连接词: that, whether, if (只连接,不作成分)
1、It is necessary (a、) that ...
2、 It is known -ed分词that ...
3、It is a pity (n、)that ...
2、连接代词: what (whatever), who (whoever), whom, whose,which (whichever)
3、连接副词: when, where, why, how
例句：It is doubtful why his parents are determined so persistently to sendthe young boy abroad for further study
(1):引导主语从句得连接词不可省略
例句: That the candidate was selected as the Party Chairman madeus excited
(2): 引导同位语从句得连接词不可省略
例句: We worried about the forecast that there would be only a few affordable apartments prepared for blue-collar workers in tier-onecities (一线城市 ) in the following three years
(3): 表示"就是否”，只用whether不用if得情况
1、后有“or not”
例句: It is unknown to all whether the new policy can improve thestandard of people's lives or not
2、引导表语从句
例句: The keynote is whether children's safety can be guaranteed
3、引导主语从句并在句首
例句: Whether the experiment could be conducted (or not)remained unknown to the students
4、作介词宾语
例句: The child is fearful of whether another war will break out at any moment
