Grammar
The Passive Voice
A. 主动语态表示主语是动作 _______。
B. 被动语态表示主语是动作 _______。
We planted the tree.
The tree was planted by us.
执行者
承受者
(1)不知道或没有必要说明动作执行者是谁。e.g. Some new computers were stolen last night.    This book was published in 1981.(2) 强调动作承受者, 而不强调动作        执行者。e.g. The window was broken by Mike.　 This book was written by him.