原因如下：
1.第二次提到某物 This is a book.The book is mine.
2.谈话双方所知道的事物Open the window ，please.
3.世界上独一无二的事物the sun，the moon，the earth
4.方向The sun rises in the east.
5.江河湖海the Yellow River the West lake
6.乐器the piano，the violin
7.一个名词被一个介词词组修饰，把介词词组放在名词的后面，the放在名词的前面.
The box on the desk is hers.The girl in red is her sister.
8.形容词的最高级和序数词前February is the second month of the year
The Changjiang River is the longest river in China.
9.用于单数可数名词前表示一类人和事物.The apple is a kind of fruit.
