We need a clear understanding of the problems involved.

　　我們需要了解清楚所涉及的各種問題。

　　This research has done much to advance our understanding of language learning.

　　這項研究大大提高了我們對語言學習的認識。

　　Stop fussing around and find something useful to do!

　　別瞎忙活了，找點有用的事干！

　　Your local library will be a useful source of information.

　　你們當地的圖書館就是很好的資料來源。

　　It's a useful introduction to an extremely complex subject.

　　這是對一門極為復雜的學科的有益入門教程。