It has been proved that his theory is practical 怎么变为 His theory has been proved ( )

问题描述:

It has been proved that his theory is practical 怎么变为 His theory has been proved ( )
如题

to be practicalpractical 不是形容词吗?那不用改为ed形式?to be practical 就对了。不是verb,不用加ed的哦。