被动语态的主要用法
被动语态中,主语是动作的承受者,主要用于以下几种情况：
1.不知道动作的执行者是谁.These books are written for children.The window was broken last night.
2.没有必要指出动作的执行者是谁.
More trees must be planted here every year.The streets should be cleaned twice every day.
3.需要强调或突出动作的承受着（接受者）.Chinese is spoken by more and more people around the world.
The old and the children must be taken good care of in China.
4.动作的发出者不是人.
Many houses were washed away in the flood.Many tall buildings were destroyed in the earthquake.