What is an idiom? An idiom is a group of words which, when used together, has a different meaning from the one which the individual words have. For example:
什么是成语呢？一个成语就是一组词，当结合在一起使用时，其意与单个词语所表达的意思均不相同。例如：
-How do you know that Sid and Nancy have separated
你怎么知道席德和南希分手了？
-I heard it on the grapevine.
我是从小道消息听到的。
Of course, the second speaker does not mean he heard the news about John by putting his ear to a grapevine! He is conveying the idea visually of information spreading around a widespread network, similar to a grapevine.
We use idioms to express something that other words do not express as clearly or as cleverly. We often use an image or symbol to describe something as clearly as possible and thus make our point as effectively as possible. For example, "in a nutshell" suggests the idea of having all the information contained within very few words. Idioms tend to be informal and are best used in spoken rather than written English.
我们使用成语是要表达其它词语无法如此清晰或如此巧妙表达的东西。我们经常会运用比喻和象征以便尽可能清晰地描述事物，因而可以尽可能有效地阐述我们的观点。比如，“in a nutshell”表明的意思是，用很少几个词来涵盖所要表达的全部意思。成语往往是非正式的，最好用于口语，而不要用在非书面表达中。
One of the best ways to learn an idiom is by looking at the context in which it is used. This can be done by concentrating on the rest of the sentence and try to guess the meaning. Many idioms are not that difficult to understand when considered in their context. For example:
We are going to have a surprise party for Tom tomorrow. It's a secret so please don't let the cat out of the bag.
明天，我们要给汤姆举办一个惊喜聚会。这是一个秘密，因此请不要泄露秘密。
'Let the cat out of the bag' is an idiom. Imagine you don't know what this idiom means; by looking at the words preceding it should be easy to guess that the speaker does not want you to tell Tom about the surprise party. Therefore, 'let the cat out of the bag' must mean something like' reveal a secret' or 'tell a secret'.
“Let the cat out of the bag”是一个成语。设想你不知道这个成语的意思；如果看看之前的单词，就会很容易地猜出，讲话者是不想让你跟汤姆谈及惊喜聚会的事情。因此，“let the cat out of the bag”一定是“揭露秘密”或“泄露秘密”之类的意思。
Other examples:
其它例子：
He was on the carpet last week for being late to work three times.
上星期，他由于迟到三次而受到批评。
It was a very long report. It took me three hours to wade through.
那是一个很长的报告。我花了三个小时才总算读完它。
Let's call it a day. I'm very tired and I think we have covered the main points of the meeting already.