The German equivalent for idiom is Spracheigentuemlichkeit, a typically long word which translated back into English means a peculiarity or distinctiveness of language. This is a good definition of idiom and helps us to understand why can idioms cause problems for language learners. There is very often no way of guessing the meaning of an idiom simply by looking at the individual words it contains. In fact, this is what can make idioms all the more confusing. For example, the learner may know the words get and goat, and wonder why she doesn't understand the sentence You get my goat! In fact, she has little chance of guessing, out of context, that it means You irritate me!
The common English expression Keep your hair on! doesn't mean you are telling someone not to have their hair cut or to hold on to their wig. It simply means Keep calm! or Don't get excited! It is true that all languages have idioms, but English seems to have more than its fair share - another reason why it is a difficult language for the more advanced learner.
Advanced learners, of course, not only want to understand the idioms they hear or read, but they also want to use them themselves. And this is where it gets really difficult! To use idioms correctly and appropriately takes many years of experience with the language. An inappropriately-used or incorrectly-used idiom will simply draw attention to the fact that the speaker is not a native speaker. This is unfortunate since the ultimate goal of most learners of English to reach a level of language indistinguishable from that of the native speaker.
As an example of what I mean, consider the idiom It's raining cats and dogs (it's raining very heavily), which many learners encounter early on in their exposure to English. What they don't learn, however, is that the expression seems to have disappeared from everyday language. In fact, I don't recall ever hearing anyone except a non-native speaker use the expression.
So appropriateness of usage is one problem; the other problem is the correctness of usage. In most idioms, changing a single word can produce a comic effect to the native-speaking listener. Telling your conversation partner that It's raining dogs and cats or It's raining cats and pigs would certainly cause a smile. The idiom Give me a hand (help me) requires the indefinite article. Give me your hand would be interpreted literally, and Give me the hand sounds ridiculous. Conversely, the idiom He needs taking in hand (somebody needs to control his behaviour) must not be used with an article. He needs taking in the hand and He needs taking in a hand would simply expose the speaker's lack of full command of English.
A related problem is the fact that some idioms are fixed in one tense or form, while others allow much more variation. For example, the idiom It's raining cats and dogs seems to be relatively fixed. To try and change the tense produces language that seems very strange indeed. None of the following variations are possible:
Similarly, the idiom You get my goat seems to be fixed. The following variations would simply cause amusement in the listener, not admiration for the speaker's command of English!
Other idioms do allow a certain amount of change. The idiom with cats and bags, for example, allows the following variations:
Many English idioms contain animal words or words for a part of the body. A selection of them is below. If you don't know them already, take a guess at their meanings and then check to see how close you were.
Are the idioms Keep your hair on! and Don't try to pull my leg! fixed or do they allow any variation?
Keep your hair on! seems to be a fixed idiom that can only be said at the time that the person you are speaking to is starting to get excited or agitated. It sound strange, for example, to say:
The idiom with legs being pulled, on the other hand, seems relatively flexible. All of the following are perfectly acceptable in everyday English:
Note also: If you suspect that someone is trying to deceive you or play a joke on you, you can say Pull the other one! i.e. Pull the other leg! (I know what you are up to!)
[One thing that makes idioms a little easier to deal with is the fact that they usually only have one meaning. Phrasal verbs are another kettle of fish (an idiom that simply means different); phrasal verbs can have several alternative senses.]