双语例句：
Stop fussing around and find something useful to do!
别瞎忙活了，找点有用的事干！
Your local library will be a useful source of information.
你们当地的图书馆就是很好的资料来源。
It's a useful introduction to an extremely complex subject.
这是对一门极为复杂的学科的有益入门教程。
It's useless to fire away with such trivial details.
喋喋不休地谈这些琐碎细节是无用的。
When it comes to getting things done, he's useless.
一涉及做事，他便不中用了。