"The cellphone was broken by my younger brother."
I don't understand why "用壞" is used here as opposed to just "壞". The former term isn't in my dictionaries and one would think that simply using the verb "to break" would be adequate.
Maybe 用坏 is transitive (i.e. requires an object – to break something) and 坏 is intransitive (i.e. cannot have an object – to break/stop functioning).
It must be because the subject is 弟弟, not 手机。弄坏 should also be acceptable.