1. 同位语从句跟在名词后面,进一步说明该名词的具体内容。引导同位语从句的名词主要有 fact, news, promise, idea, truth等。连接词用that (不用which)及连接副词how, when, where, why等。

　　2. 关联词只能用whether不能用if表示“是否”的情况如下:

　　1)在表语从句和同位语从句中。

　　The question is whether the film is worth seeing.

　　The news whether our team has won the match is unknown.

　　2)在主语从句中,只有用it作形式主语时,whether和if都能引导主语从句,否则,也只能用whether。

　　Whether we shall attend the meeting hasn't been decided yet.

　　It hasn't been decided whether (if) we shall attend the meeting.

　　3)在介词之后(介词往往可以省略)

　　It all depends (on) whether they will support us.

　　4)后面直接跟动词不定式时。 He doesn't know whether to stay or not.

　　5)后面紧接or not 时。 We didn't know whether or not she was ready.

　　6)引导让步状语从句,只能用whether。 Whether you like it or not, you must do it well.

　　7)用if会引起歧义时。

　　Please let me know if you like it. 该句有两个意思：“请告诉我你是否喜欢”.或“如果你喜欢,请告诉我。”用whether就可以避免。

　　3. 在下面几种情况下必须用“that”引导定语从句：

　　1)先行词是不定代词：all, few, little, much, something, nothing, anything等。 All that we have to do is to practice every day.

　　2)先行词被序数词或形容词最高级所修饰。 The first lesson that I learned will never be forgotten.

　　3)先行词被all, any, every, each, few, little, no, some, 等修饰。 I have read all the book (that) you gave me.

　　4)先行词被 the only, the very, the same, the last 修饰时。 He is the only person that I want to talk to.

　　5)先行词既有人又有物时。 They talked of things and persons that they remembered in the school.

　　4. 先行词是表示地点时,要根据从句的谓语动词是及物的还是不及物的。如果是及物的就用that(which),否则用where。

　　This is the house where he lived last year. This is the house that (which) he visited last year.