参考答案：

An idiom is a construction, expression, etc. having a meaning different from the literal one or not according to the usual patterns of the language.

Idioms are difficult to learn because their meaning is unpredictable and their patterns may be unusual. Moreover, they are culture rich items. To learn them means to learn not only language but also the under-lying culture. These are the reasons why idioms are hard to understand and harder to use correctly.