㈠ 表示世上唯一的事物，如：

① the sun; the earth; the sky; the North Pole

㈡ 当一个名词被再次提起时，如：

② A car knocked against a tree. We can still see the mark on the tree made by the car.

㈢ 当名词后面有个修饰作用的短语或分句时，如：

③ The girl in red is my sister.

④ The thief who stole your wallet yesterday was arrested.

㈣ 加在某些形容词前面，以表示某类人或事，如：

⑤ The rich often get richer and the poor, poorer.

⑥ The doctor lost no time in giving help to the injured.

⑦ Don't expect the impossible.

㈤ 加在某些山川、河流、岛屿、国家名称之前，如：

⑧ the Alps; the Thames; the British Isles; the Philippines; the Atlantic Ocean.