一、被动语态用于以下几种情况
1. 不知道或没有必要指出动作的执行者。如：
Many new buildings are built in our city.
2. 强调动作的承受者。如：
The wounded man has been sent to a hospital.
3. 动作执行者有比较长的修饰语。如：
The person was supported by those who wished to have a chance to make friends with him.
4. 出于礼貌，不愿说出动作的执行者。如：
You are requested to attend the party.
二、过去分词作表语与被动语态的区别
1. 系表结构中的过去分词是表示主语的特征或状态；而被动结构则是表示主语是动作的承受者。试比较：
Our building is surrounded with the trees.
The film is usually showed on Sunday.
2. 系表结构不能带有by引导的短语；而被动结构中可以用by引导的短语引出动作的执行者。试比较：
The book is well written.
The book was written by Lu Xun.
[中考英语被动语态解析]