keep、sell之类的词什么时候用被动形式,什么时候用主动表被动?我现在分不清了.

一、sell做不及物动词时,就是它后面没有接宾语的时候,常常用主动表被动.
e.g.The kind of book sells well.
sell做及物动词时,就是它后面接宾语的时候,会用典型的被动语态表被动.
e.g.He sold his house.
His house was sold by him.
二、keep同sell的道理一样.