1. 同位语
若两个指同一人或事物的句子成分放在同等位置，其中一个句子成分可被用来说明或解释另一个句子成分，那么后者就叫做前者的同位语。如：
Pele, the famous soccer player, grew up in Brazil. 足球明星贝利是在巴西长大的。(Pele和the famous soccer player指的是同一个人，后者用以)
2. 同位语从句
当名词后面所接从句是用以说明该名词的实际内容时，这个从句就是同位语从句；用以引导同位语从句的最常用的连词是that。如：
The news that we are having a holiday tomorrow is not true. 明天放假的消息不实。
The idea that you can do this work without thinking is wrong. 你可以不动脑筋做此工作的想法是错误的。
有时也可以用whether、连接代词或连接副词等来引导。如：
It is a question how he did it. 那是一个他如何做的问题。
Do you have any idea where he lives? 你知道他住在哪里吗?
She had no idea why she thought of him suddenly. 她不明白她为什么突然想到了他。
We are not investigating the question whether he is trustworthy. 我们不是在调查他是否可信赖的问题。
注意，whether可以引导同位语从句，但是与之同义的if则不能。如：
你回答我的问题：你来不来。
误：Answer my question if you are coming.
正：Answer my question whether you are coming.
受同位语从句修饰的常见名词
能后接同位语从句的名词不多，常见的有answer, belief, conclusion, decision, discovery, doubt, explanation, fact, hope, idea, information, impression, knowledge, law, message, news, opinion, order, possibility, problem, promise, proof, proposal, question, reply, remark, report, risk, rumor, story, suggestion, thought, truth, theory, thought, wish, word等。如：
The hope that he may recover is faint. 他复原的希望是渺茫的。
There was little hope that they would survive. 他们幸存的希望很小。
At first I was of the opinion that you were right. 开始我认为你是对的。
Have you any proof that he is thief? 你有没有证据证明他是小偷?
I got the impression that you are unhappy. 我有印象你不太愉快。
I received a message that she would be late. 我得到的信息说她可能晚到。
