用在世界上独一无二的事物前。

如：explore the surface of the moon.

用在序数词、形容词的最高级前。

如：this was the first step towards achieving a peace agreement.

用于上文已经提到过的人或事物。

如：he bought a new car.the car was very beautiful.

用在表示方位、乐器和影剧院的名词前。

如：the wind is blowing from the east.

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.