同位语在句中用于说明或解释主语或宾语的某一属性或特征。同位语分为主语同位语和宾语同位语。主语同位语有三种位置：1. 名词、主系表结构，位于句首逗号前的孤立名词即为主语同位语，可以确定主语。例如：A prolific writer of prose and verse, Helen Hunt Jackson took up the cause of Native Americans in her novel, Century of Dishonor。2. 主语、名词、动宾结构，名词位于主语后，与动宾结构并列。例如：The tongue, the chief organ of taste, is an important aid in chewing and swallowing。3. 主语同位语+主谓宾结构，名词与主谓宾结构直接相连，无逗号分隔。例如：The teacher Mick has taught TOEFL for six years。宾语同位语有两种位置：1. 主语、谓语、宾语后，位于句尾的孤立名词必是宾语同位语。例如：By 1872 the United States had 70 engineering colleges, an astonishing expansion credited largely to the Morrill Act of 1862。2. 主语、谓语、宾语后，再跟宾语同位语。例如：In 1952 Ernest Hemingway published The Old Man and the Sea,a work that won him the Nobel Prize for Literature in 1954。
