英语语态指的是主语与谓语动词之间的关系，如果是主动关系（主语是动作的执行者），句子就是主动语态；如果是被动关系（主语是动作的承受者），句子就是被动语态。
使用被动语态的情况
1. 我们不知道或没有必要说明动作的执行者是谁的时候(这时不带by引起的短语)。
Printing was introduced to Europe from China. (省略了by短语。)
Look! There is nothing here. Everything has been taken away. (省略了by短语。)
2. 当我们出于礼貌、措辞委婉等方面的考虑不愿说出动作的执行 者是谁时。
You are requested to give a performance.
You are cordially (诚挚地) invited to a party to be given a t the Teachers' Club at 3 p.m. Nov. 23.
3. 当我们强调或兴趣在动作的承受者时(这时可带by引起的短语)。
The song was composed（组成; 写作）by a student.
A good time was had by all.
