徒劳无功 --------- work to no avail

例句：
A: I have helped him so much but it works to no avail.
我帮了他那么多,但却徒劳无功。
B: Why do you think so?
你为什么这么想呢？

同义词：
make a futile effort
work in vain
be a fool for one's pains