Usage patterns
愚かな〈人〉a foolish person
He is truly a foolish person.
愚かな〈事〉をするto do something foolish
Don't do something so foolish.
〈事〉のは愚かだit is foolish to do (something)
It was foolish to trust him.
Expressions
言うも愚かit goes without saying; needless to say