It's raining cats and dogs.
Idioms are fun and useful expressions that usually cannot be understood by defining the separate words. For example, if your host mother says, "I think it's time to hit the hay", she means, "I think it's bed-time"! You would not be able to understand her by looking up the words hit and hay. Like phrasal verbs, these expressions need to be memorized as a whole. Try visualizing them or drawing pictures when you study them. For example, draw a picture of a cow sleeping in some hay. The idiom out of the blue means "unexpectedly". Your teacher or homestay family will call you one smart cookie if you come up with an idiomatic expression like that when they least expect it!
Most idioms don't seem to make any sense because their origins are so old. Some of them come from ancient literature or even classic films. Learning the background of idioms can help you to remember them. Try checking your dictionary or asking a teacher or native speaker if they know the origin.
The following pages have common idioms in specific categories. Each idiom has its meaning and an example sentence. There is also a quiz for each category to check your understanding. After reading these pages, the quizzes should be a piece of cake for you!