被动语态变主动语态怎么变

This factory produces machine tools. 这家工厂生产车床。
>>> Machine tools are produced in this factory. 车床是在这家工厂生产的。
They found a wallet in the car. 他们在车里捡到一个钱包。
>>> A wallet was found in the car. 钱包是在车里捡到的。
They are repairing the bridge. 他们正在修复桥梁。
>>> The bridge is being repaired. 桥梁正在修复中。
They often make fun of her. 人们常拿她开玩笑。
>>> She is often made fun of. 她常被人取笑。
He made a mess of the house. 他把房子搞得乱七八糟。
>>> The house was made a mess of. 房子被搞得乱七八糟。
He took great care of his books. 他非常爱护书籍。
>>> His books were taken great care if. 他的书籍受到精心爱护。
>>> Great care was taken of his books. 他的书籍受到精心爱护。

比方说:
I have been to Newyork.可以变成:I went to Newyork.
主要是把have done改成did就可以了....

原句：蛋糕被我吃了
改后：我把蛋糕吃了or我吃了蛋糕

明白？

你要举个例子啊