同位语从句一般用that,whether，what, which,who, when, where, why, how 等词引导，常放在fact, news, idea, truth, hope, problem, information, wish, promise,answer, evidence, report, explanation, suggestion, conclusion等抽象名词后面，说明该名词的具体内容。换言之，同位语从句和所修饰的名词在内容上为同一关系，对其内容作进一步说明。 

1. 如同位语从句意义完整，应用that引导同位语从句。（即that不充当任何成分，只起连接作用，不可省略）

The general gave the order that the soldiers should cross the river at once.

将军下达了战士们立即过河的命令。

2. 如同位语从句意义不完整，需增加"是否"的含义，应用whether引导同位语从句。（if不能引导同位语从句）

We'll discuss the problem whether the sports meeting will be held on time.

我们将讨论运动会是否会如期举行的问题。

3. 如同位语从句意义不完整，需增加"什么时候"、"什么地点"、"什么方式"等含义，应用when, where, how等词引导同位语从句。

I have no idea when he will beback.

I have no impression how he wenthome, perhaps by bike.