1.不知道动作的执行者是谁。
如：
This watch is made in China.
2.没有必要指出动作的执行者是谁。
如：
More trees must be planted every year.
3.需要强调或突出动作的承受者时。
如：
Chinese is spoken by more and more people in the world.
4.动作的发出者不是人而是物时。
如：
Many flowers are planted by them.
1.一般谓语动词主动语态变为被动语态
2.带双宾语的谓语动词变为被动语态
3.被动语态后动词形式的选择：主动语态中感官动词see, hear, watch, feel, notice等和使役动词let, make, have等词后跟省略to的不定式，变为被动语态时要加上不定式符号to。
如：
We often hear a girl read English in the evening.(变为被动语态)
A girl is often heard to read English in the evening by us.
1.open, lock, write, read, sell, clean, wash等词作不及物动词且它们的主语为物时，可用主动语态表示被动意义。
如：
The pen writes very smoothly. (这支钢笔写起来很流畅)
This sweater washes well.（这件毛衣很耐洗）
2. 感官系动词look,sound,taste,smell,feel用主动形式表被动意义。
如:
School uniforms look good on us.（我们穿着校服很好看）
Dinner smells delicious.(晚饭闻起来很香)
3. 固定句型表示被动意义。
如：
①need doing sth. “需要被做”
The watch needs repairing.
这块表需要被修理。
②want doing sth. “想要被做”
The hair wants cutting. 该理发了。（头发想要被理）
③require doing sth. “需要被做”
English requires reading more often.
英语应该被常读。
④be worth doing sth. “值得被做”
This book is worth reading.
这本书值得被读。
1.被动语态表示一个动作，句子的主语是动作的承受者；而系表结构则表示主语的特点或所处的状态。如：
This shop is closed at nine o’clock. (动作) 这家商店九点关门。
This shop is closed today. (状态) 这家商店今天不营业。
2.系表结构通常只有一般现在时和一般过去时，而被动语态结构则可用于多种时态。
注：若宾语为非谓语形式，则通常不变为被动语态。
如：
My mother enjoys watching TV.(主动语态)
Watching TV is enjoyed by my mother. (错误)
I want to have a try.(主动语态)
A try is wanted by me.(错误)
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