We need a clear understanding of the problems involved.
我们需要了解清楚所涉及的'各种问题。
This research has done much to advance our understanding of language learning.
这项研究大大提高了我们对语言学习的认识。
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。