定冠词的用法如下：

1．指彼此都知道的人或事物，如：
      Open the door, please.
      Go and close the window.

2．特指某（些）人或某（些）事物，如：
      The girl in red is my sister.

      The man over there is our English teacher.

      The book on the desk is mine.

3．指前面提到过的人或事物，如：

      Here is a picture of a modern car factory. My father works in the factory.

4．用在序数词和形容词最高级前,在方位词前。如：

     The Nile is the longest river in the world.

     He won the first prize.

     The sun rises in the east.

5．用在某些名词化的形容词，过去分词前，表示一类人或事物， 如：

     the rich, the sick, the new , the false

6．用在某些专用名词前， 如：

     the Great Wall

     the People’s Republic of China

     the United Nations

     the Netherlands

7．在江河、海洋、山脉、湖泊、群岛的名称的前面，如：

     the North China Plain

     the Rocky Mountains

     the Yangtse River

8．在一些用语中，如：

     in the morning(/afternoon/evening), in the day, by the way, on the whole, the next morning, all the year around, the other day, at the same time