正常情况下的宾语变成主语时,可以认为是被动语态.
如： He wrote a letter yesterday. a letter是主动语态的宾语.
A letter was written (by him) yesterday.a letter成为主语,它不会是自己写的,所以是被动语态.
They have cut down the tree.the tree是宾语.
The tree has been cut down (by them).the tree成为主语,它不会自己砍的,所以是被动语态.