不知道动作由谁发出,或由于某种原因没有必要说明谁发出动作。如: This table is made of wood. 需要突出或强调动作的承受者时,如: This park was built for children.
注意:主动句中的主语如果是people,they,somebody等含糊的表示“人或人们”,没有确指执行者是谁,为被动句时,通常省略“by+执行者”。如:The door was opened secretly. But nobody came in.
注意:在主动句里,不定式在make,see,hear等动词后面作宾语补足语时都不带to,但变成被动句时,后面的不定式都需带to。如:He was made to do that work.
主动语态不能变被动语态的情况:学了被动语态,别以为主动句和被动句可以随意转换,千万要注意呀!有些主动语态不能转换成被动语态。当宾语是反身代词时, 如:You should take care of yourself. 当谓语是表状态的而不是表具体动作的及物动词时,如:Does the skirt suit you?