A．that 连词that本身没有意义，在从句中不担任任何句子成分，在宾语从句中有时可省略。 I hear (that) he has joined the football club. 我听说他已经加入了足球俱乐部。 That light travels in straight line is known to all. 众所周知，光是以直线传播的。 It so happens that I know the man. 碰巧我认识那个。 Is it certain that they will win 他们一定会赢吗
B．whether和if 连词whether和if本身有意义（解释是否），在从句中不可省略。 1．whether可以连接所有的名词性从句，而if只能引导宾语从句。 I didn't know whether he would attend the concert. 我并不知道他是否参加音乐会。（宾语从句，可用if代替whether） The question is whether it s worth trying. 问题是值不值得试一试。（表语从句，不可用if代替whether） Whether she comes or not makes no difference. 她来不来都没有关系。（主语从句，不可用if代替whether） He must answer the question whether he agrees to it or not. 他必须回答他是否同意此事这样一个问题。（同位语从句，不可用if代替whether） 2．whether引导的宾语从句可作介的宾语，而if则不能。 It all depends on whether they will support us. 这完全取决于他们是否支持我们。(不用if) Ryan was worrying about whether he had hurt her feelings. 瑞恩担心是否伤害了她的感情。 3．if既可引导语从句，也可引导宾语从句。如果用if会引起歧义，应避免使用if，而用whether。 Please let me know if you want to join us. 请告诉我你是否想加入我们。（if引导的从句可被看成是宾语从句，if表示是否） Please let me know if you want to join us. 如果你想加入我们的话，请告诉我一声。（if引导的从句可被看成是条件状语从句，if表示如果） Please let me know whether you want t join us. 请告诉我你是否想加入我们。（为避免引起歧义，可用whether表示是否）
C．who, whom, whose, what, which 连接代词who, whom, whose, what, which等在从句中既起连接作用，同时又担当主语、宾语、定语、表语等成分。 No one knows who he was waiting for. 没人知道他当时在等谁。 We are worrying about what we should do next. 我们正在为下一步该怎么办而烦恼。 Tell me whos house it is. 告诉我这是谁的家。 Let me know which train you will be arriving on. 告诉我你将乘哪列火车到达。 D．where, when, how, why 连接副词where, when, how, why等在从句中既是连接词，又作状语。 I don't know where we are going to have the meeting.我不知道我们将在哪儿开会。 She always thinks of how shecan work well. 她总是在想怎样能把工作做好。 What I am anxious to know is when we can visit the museum. 我急于知道我们什么时候能参观博物馆。 比较： when引导的时间状语从句和名词性从句中的不同时态。 Please lt me know when you arrive. 你到的时候，请告诉我一声。（when引导的是时间状语从句，从句中用一般现在时来表示将来。） Please let me know when you will arrive. 请告诉我你什么时候到。（when引导的是宾语从句，从句中用一般将来时。）
E．whoever, whomever, whatever, whichever, whosever 连接代词whoever = anyone who任何人, 无论谁；whatever =anything that凡是...，无论什么；whichever = anything that无论那一个, 任何一个；whomever = anyone whom （whoever的宾格形式）。 Whoever comes to the club is welcome. 不论谁来参加这个俱乐部都欢迎。 Whatever is worth doing at all is worth doing well.凡是值得做的就值得做好。 They ate whatever they could find o the deserted island. 他们在荒岛找到什么就吃什么。 You may do whatever you will. 你可做任何你想做的事。 I'll take whichever book interests me.我愿买任何让我感兴趣的书。 Whosever book is overdue will be fined. 不管谁的书过期未还都要被罚款。 比较： 连接代词whoever在宾语从句中作主语，whomever作宾语从句中宾语。 You may offer he book to whoever wants it. 你可把这本书给任何想要的人。（此句中不能使用whomever，因为whoever在宾语从句中作wants it的主语） You may offer the book to whomever you like.你可把这本书给任何你喜欢的人。（whomever在宾语从句中作you like的宾语）
