當一個名詞(或其它形式)用來限定或説明另一個名詞或代詞時，這個名詞(或其它形式)就是同位語。
同位語與被它修飾的詞在格式上要保持一致，而且同位語一般都是緊跟所修飾的詞語的。
同位語與所修飾的詞語關係緊密時，不用逗號隔開；同位語對其所修飾的詞語只作補充解釋時，可用逗號隔開。如：
1) My sister Mary will visit me tomorrow morning.
2) Last Saturday, I met my old friend, James Brown.
第1)句中的Mary就是my sister的同位語，第2)句中的James Brown就是my old friend的同位語，指的都是同一個人。
They were worried over the fact that you were sick. 他們為你生病發愁。
I received a message that she would be late. 我得到的信息説她可能晚到。
The news that we are having a holiday tomorrow is not true. 明天放假的消息不實。
The idea that you can do this work without thinking is wrong. 你可以不動腦筋做此工作的想法是錯誤的。
當一個句子用來充當某個名詞或代詞的同位語時，這個句子就是該名詞或代詞的同位語從句。
同位語從句主要用來對其前面的抽象名詞進行解釋説明，被解釋説明的詞和同位語在邏輯上是主表關係。如：
The idea that we will visit Paris next week excites every one of us.
句中的同位語從句we will visit Paris next week與the idea之間就是主表關係，即同位語從句和它所修飾的成分之間其實可以用系動詞連接起來。如：
The idea is that we will visit Paris next week.
Soon came the order to start the general attack. 很快下達了發起總攻的`命令。（to start the general attack與the order同位）
He followed the instruction to walk along a certain street where I picked him up. 他照吩咐沿某一條街走，我在那裏接他上了車。（to walk along…與the instruction同位）
(一)名詞或名詞短語作同位語
Jack, the mayor, will attend the meeting next Friday.
句中的the mayor作Jack的同位語。
Jerry, our beloved pet dog, has lived with us for eight years.
句中的our beloved pet dog作Jerry的同位語。
(二)直接引語作同位語
Now let's get down to the girl's question, “Who will take over the job?”
句中引號的直接引語就作為question的同位語。
(三)句子作同位語
I don't believe the fact that she is a spy.
句中的she is a spy作the fact的同位語。
We none of us said anything. 我們誰也沒説話。
We have none of us large appetites. 我們誰飯量都不大。
They neither of them wanted to go. 他們兩人都不想去。
They've neither of them succeeded in winning her confidence. 他們兩人誰也沒能贏得她的信任。
【注】同位語並不影響其後句子謂語的“數”，如：
學生每人都有一本詞典。
正：The students each have a dictionary.
誤：The students each has a dictionary.
請比較下面一句（謂語用了單數，因為each為句子主語）：
正：Each of the students has a dictionary.
3、代詞we, us, you等後接同位語
Are you two reading? 你們二人在看書嗎？
They three joined the school team. 他們3人蔘加了校隊。
She has great concern for us students. 她對我們學生很關心。
He asked you boys to be quiet. 他要你們男孩子安靜些。
We girls often go to the movies together. 我們女孩子經常一起去看電影。
The current affair, the biggest in its history, is being held in Guangzhou. 目前的交易會，是有史以來規模最大的，正在廣州舉行。
He read all kinds of books, ancient and modern, Chinese and foreign. 他讀了很多書，古今中外都有。
People, old and young, took to the streets to watch the parade. 老老少少的人都來到街頭觀看遊行。
【注】這類同位語與定語比較接近，可轉換成定語從句。如：
The current affair, the biggest in its history, is being held in Guangzhou.
=The current affair, which is the biggest in its history, is being held in Guangzhou.
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