根据很多语法书上的解释，主动语态与被动语态的主要区别之一是强调的内容不同。真的是这样的吗？请大家试着把下面的句子变为被动语态，看是不是只是强调的内容不同？

Every schoolboy knows one joke at least.

John cannot do it.

Mary can’t teach Peter.

Michael wouldn’t ride the white horse.

Smoking causes lung cancer.

Michael ate the apple.

Michael often eats apples.

下面的句子可以变为被动语态吗？

We often see each other.

The man shook his head.

We changed the bus on the way home.

He read the book.