1、用在世界上独一无二的事物前。
如：Explore the surface of the moon.
2、用在序数词、形容词的最高级前。
如：This was the first step towards achieving a peace agreement.
3、用于上文已经提到过的人或事物。
如：He bought a new car was very beautiful.
4、用在表示方位、乐器和影剧院的名词前。
如：The wind is blowing from the east.
5、用在江河、湖泊、海洋、山脉、群岛、沙漠和海湾等的名称前。
如：the Yellow River，the Pacific Ocean，the Alps.
