What are idioms?
'Idiom' is not originally an English word – it is one of the many that have come into the language from Greek. 'Idiom' means 'one of a kind' and indicates that a phrase is being used with a special meaning that can be very different to the literal meaning. If someone says 'When Mr Bloggs fell over the files someone had left on the floor, he hit the roof.' we can understand that though Mr Bloggs actually hit the floor, he was extremely angry about it; because 'hitting the roof' is an idiom for being very angry and letting everyone know it. Idioms are a problem for language learners because they have to be learned individually, they are often ungrammatical, and English people often assume that their listeners know the idiom, and make a joke or a pun on it. For example 'When me Bloggs fell over the files someone had left on the floor, he went ballistic.' (That is, he hit the roof like a rocket starting on its journey.)
Why are idioms used?
Idioms are not only used, they are used a lot. Almost every English person uses several idioms in the course of a conversation. Sometimes an idiom is used as a short way of expressing a more complicated idea. For example if you call something a parson's egg this is a quick way of saying that there are good parts and bad parts to something, but overall it is not satisfactory. Also idioms help to make English a more colourful language. If you say 'learning a language is an uphill task', anyone who has walked or ridden a bicycle up a steep hill will immediately understand the effort involved, because idioms sometimes bring a clear mental picture to mind. Say that something is a drop in the ocean and your listener knows at once that this is a very tiny amount indeed.
Where do idioms come from?
Idioms often come from jargon – the technical language used by a group of specialists. For example soldiers have given us overshoot for 'to go past the target' and a 'last ditch effort' for a final try before giving up. Sportsmen have given many idioms, such as being on a sticky wicket from cricket, which means being in a difficult situation. (Today we often just call something 'sticky' because we assume everyone knows the full idiom.) Because the British used to be a nation of sailors, many idioms have come from there – for example a ship with too many sails in a strong wind would sail erratically, so a drunk person is sometimes said to be three sheets to the wind.
Other idioms are more logical – if a tanned person is scared, the blood goes from his face, which is why a coward is called yellow', and it is not hard to understand why a person in a bad mood is like a bear with a sore head.
How do you recognize an idiom?
Idioms are recognizable because the literal meaning might not make sense. If someone says they will turn over a new leaf after getting into trouble, then we should not ask ourselves why they are taking up gardening but assume that the phrase has an idiomatic meaning (to make a start on becoming a better person). So if someone if you hear of a person who tries to pass the buck about being a bull in a china shop because he was catty about the apple of someone's eye, you can assume that very idiomatic language is being used.
Should you use idioms?
Use idioms with care, and only when you are sure of their meaning. Some idioms are very colourful, but they can also express strong feelings, and it is easy to give offence without meaning to. If you say someone has a light touch you are praising that person's ability to run things without interfering, but if you say that person is light-fingered, you are calling him a thief. A person who does not give away secrets may be poker-faced but if he is tight-lipped he is angry but saying nothing.
Note also that in 21st century England almost the only people who say that it is raining cats and dogs are pensioners and language students. A modern Englishman will more probably remark 'It's chucking down'. Some idioms last for generations, but others come in and out of fashion in a year or less. So be careful, or your idiom use will go all pear-shaped. (Turn out badly.)
Why should you learn idioms?
Of course. Learning idioms is not a piece of cake(very easy) but once you know them they can be a lot of fun, and anyway, because English people use idioms non-stop you will be all at sea(totally confused) in most conversations until you learn the ropes(understand how things work)
How do you learn idioms?
Reading English texts or listening to native speakers will help you to find you a large number of idioms. After a while some idioms will become familiar, and you will learn the places and situations where you hear them. English newspapers use idioms a lot, but they often make puns of the idioms rather than using the original idiom. For example a famous newspaper critic wrote of an actor 'he went through the range of emotions from a to b' (instead of 'a to z' which means 'all of them'.) So you might prefer to start by reading books, listening to conversations, and of course, doing the exercises here.
So roll up your sleeves (get ready for a job) and put your noses to the grindstone! (Start working hard.) It's time to get cracking (get started).