若从句来源于一个陈述句,那么,连词用that,在口语中that可以省略
They believe that the computer will finally take the place of human beings.(他们相信计算机终将代替人类。)
若从句来源于一个一般疑问句,连词则用if 或whether
I wonder whether I should say something for him to the headmaster.
(我不知道是不是该为他在校长跟前说点什么。)
(从句来源于一般问句Shall I say something for him to the headmaster?)
若从句来源于一个特殊疑问句,则连词就是疑问词(如what,who,where,when等)
He asked me where he could get such medicine.
(他问我在哪儿能搞到那样的药。)
(从句来源于特殊问句Where can he get such medicine? )