He is one of those people who promise more than can be done.为什么用被动语态（can be done),而不是主动的(can do)

这里强调的是量， 与主句more 相对， 意即“承诺多于落实”
完整的说法是more than what can be done.
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more than 后面的主语是承诺的事情，事情不能主动去做，只能被动的做。这句话可以理解为： 他是这类说的比做的多的人之一。其实这句话也可以写成： He is one of those people who promise （to do something）more than (something) can be done.

我估计是promise后面的东西省掉了，能够被做的事情，肯定是被动。
老师祝你学习进步！
望采纳，多谢！^_^

这里指的是promise 那只能是被人们实现啊
promise 不会主动实现的

He is one of those people who promise more than can be done.
也可以变成主动语态的：He is one of those people who promise more things than they do.
好,现在来回答你的问题.He drank more wine than was good for him and got drunk
.He drank more wine than was good for him (他酒喝得太多,超出该喝的范围)
than在你的句子里面是连接词也是从句的主语,在我的句子里面也是如此.
结构是：-- - -比较级 than 谓语（+其他成分） .
又如：
1 Here in my bedroom there are more books than are needed for my school work.
2 Every day I do more homework than is good for my health.
现在,你会了吗?祝你学习天天向上.