An idiomatic expression is a phrase or saying whose meaning cannot be understood on its own - if you don't already know what it means you won't be able to figure it out. This means that dealing with idiomatic expressions are one of the hardest parts of learning a language. Because you can't figure them out, the only way to be able to deal with idiomatic expressions is to learn the most common ones off by heart.
Some idiomatic expressions are uncommon or are highly informal. Other times, idiomatic expressions form part of a local dialect and are not heard in every part of the country. As such, if you are planning on learning some idiomatic expressions it's important to make sure that you learn the most useful ones. Some of these are included below as examples.
When pigs fly
Refers to something that is highly unlikely or effectively impossible
"Swansea City will come top of the Premier League when pigs fly"
Refers to something that is certain or indisputable
"Pepsi is the best cola on the market, hands down"
Down in the dumps
When someone is upset for a long time they are down in the dumps
"She's been down in the dumps since she lost her job last month"
Raining cats and dogs
Raining especially heavily
"Deary me, it's raining cats and dogs!"
Driving me up the wall
When something is annoying someone to an extreme degree
"The noise coming from next door is driving me up the wall!"
All Greek to me
When something is impossible for you to understand
"I just don't understand American Football. It's all Greek to me."
Keep your chin up
Stay optimistic, don't let your troubles get you down
"Don't worry, keep your chin up and everything will be fine!"
Hold your horses
Slow down, be patient
Usually spoken as a sentence on its own "Woah! Hold your horses!"
All in the same boat
All in the same position, in the same situation
"It's not just you who's been fired, we're all in the same boat"
When someone is rogue or unpredictable
"Inspector Kowalski is a loose cannon, he never plays by the rules!"
• Rise and shine
• Wake up and be happy
• "Rise and shine, it's a brand new day!"
Under the weather
Down in the dumps
"I've been feeling a little bit under the weather recently"
All of the idioms used above are common around the English speaking world and in both formal and informal contexts. There are, however, many more idioms than this that you will hear spoken in everyday life. The best way of learning is just by getting into conversations with ordinary people. There are also many books that it is possible to buy for you to learn the most important idioms in the English language. The key is to find a list that is neither too long nor too short and which is relevant to the type of English that you want to learn and to speak.