有时被动语态不能转换为主动语态。这主要有下列几种情况
1.某些句子结构习惯上只用被动语态,这时就不能转换为主动语态。例如：
All bodies are made up of atoms.
The old man’s race of life was nearly run.
这种被动结构还有be alleged, be bound up with, be done up, be grown over, be knocked up, be held up, be inclined, be lost on, be obliged to, be posted, be rumored to, be said to, be transported, be wound up等。
2.以代词it作形式主语的被动句，一般不能转换为主动句。例如：
It is known that sound is a form of energy.
It is observed that the situation of the country is getting better and better.
3.当被动句的施动者没有表示出来时,不能转换为主动句。例如：
Albert Einstein was born in 1879.
Manfred was killed in the Second World War.
如果改成主动句时加上一个施动者(如they, someone等),就会有损原句的意思。
