【無駄な】
➊prove useless 無駄だと分かる - Those strategies proved to be useless.
➋completely useless まったく役に立たない - That machine is completely useless.
❸useless as A : Aとして役立たない - that is as useless as a keyboard without keys.

❹useless for ～ing: ～するのに役立たない - This technique is useless for studying.
❺useless to 人: 人に役立たない - It is useless to argue with people. They never change their minds.