1.不知道動作的執(zhí)行者是誰。
如：
This watch is made in China.
2.沒有必要指出動作的執(zhí)行者是誰。
如：
More trees must be planted every year.
3.需要強調或突出動作的承受者時。
如：
Chinese is spoken by more and more people in the world.
4.動作的發(fā)出者不是人而是物時。
如：
Many flowers are planted by them.
1.一般謂語動詞主動語態(tài)變?yōu)楸粍诱Z態(tài)
2.帶雙賓語的謂語動詞變?yōu)楸粍诱Z態(tài)
3.被動語態(tài)后動詞形式的選擇：主動語態(tài)中感官動詞see, hear, watch, feel, notice等和使役動詞let, make, have等詞后跟省略to的不定式，變?yōu)楸粍诱Z態(tài)時要加上不定式符號to。
如：
We often hear a girl read English in the evening.(變?yōu)楸粍诱Z態(tài))
A girl is often heard to read English in the evening by us.
1.open, lock, write, read, sell, clean, wash等詞作不及物動詞且它們的主語為物時，可用主動語態(tài)表示被動意義。
如：
The pen writes very smoothly. (這支鋼筆寫起來很流暢)
This sweater washes well.（這件毛衣很耐洗）
2. 感官系動詞look,sound,taste,smell,feel用主動形式表被動意義。
如:
School uniforms look good on us.（我們穿著校服很好看）
Dinner smells delicious.(晚飯聞起來很香)
3. 固定句型表示被動意義。
如：
①need doing sth. “需要被做”
The watch needs repairing.
這塊表需要被修理。
②want doing sth. “想要被做”
The hair wants cutting. 該理發(fā)了。（頭發(fā)想要被理）
③require doing sth. “需要被做”
English requires reading more often.
英語應該被常讀。
④be worth doing sth. “值得被做”
This book is worth reading.
這本書值得被讀。
1.被動語態(tài)表示一個動作，句子的主語是動作的承受者；而系表結構則表示主語的特點或所處的狀態(tài)。如：
This shop is closed at nine o’clock. (動作) 這家商店九點關門。
This shop is closed today. (狀態(tài)) 這家商店今天不營業(yè)。
2.系表結構通常只有一般現在時和一般過去時，而被動語態(tài)結構則可用于多種時態(tài)。
注：若賓語為非謂語形式，則通常不變?yōu)楸粍诱Z態(tài)。
如：
My mother enjoys watching TV.(主動語態(tài))
Watching TV is enjoyed by my mother. (錯誤)
I want to have a try.(主動語態(tài))
A try is wanted by me.(錯誤)
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