What are idioms?
Idioms are non-literal expressions. This means that the meaning that you are expected to understand is different from what the words should mean. For example, someone can literally 'hit the roof' if they fall from somewhere high, and strike the top of a house. However, most English speakers will understand that 'hitting the roof' is an idiom for getting very angry very quickly.
For example:
'What did Jim say when you told him that you had crashed his new car?'
'What do you expect? He hit the roof.'

Why are idioms used?
Idioms are a part of language (and not just a part of English). They give a meaning more vividly and often more briefly than a long literal explanation would do. For example, I have just told you that an idiom gives you all information that you need but compresses it by giving you a mental picture that explains everything, but I could have told you that idioms give you the meaning 'in a nutshell'.

Also because there is such a huge number of idioms to choose from, a person's choice of idioms tells you much about that person's character and origins. For example, if you hear the idiom 'raining cats and dogs' you are probably listening to someone elderly or a language student, since that idiom has mostly dropped out of everyday use (the modern expression is 'it's chucking it down'). However 'raining cats and dogs' is a favourite of EFL textbooks.

Where do idioms come from?
Idioms often come from jargon. 'Jargon' is the language that any group of people who work together in a particular subject develop for themselves. (For example the expression 'Reboot to clear the ram' does not mean much to someone who does not know computers.) Sometimes these expressions 'escape' into the general language, often with their meaning considerably changed.

For example 'a steep learning curve' was considered a good thing when it was jargon among teachers, because it meant a large amount of learning in a short time. However, the modern idiom means almost the opposite - something which is very hard to get to understand.

Many idioms come from sport e.g. 'Gone to ground', sea travel (the English used to do this a lot), e.g. 'taken aback' or from the body e.g. 'get cold feet'. No-one knows where some other idioms come from, but everyone has a theory!

How do you recognize an idiom?
Idioms have a fixed form. For example the idiom 'A bull in a china shop' describes a socially or literally very clumsy person. However, it must be a bull, and it must be a china shop. 'A calf in a porcelein shop' has nothing but its literal meaning. However, the English love playing with words, and often extend idioms or make up expressions which twist idioms to suit a particular situation. (Which is why British newspaper headlines are often incomprehensible to non-native speakers.)
For example:
Bill does not like Jane. He thinks she is tactless and annoying. An insulting idiom for a stupid female is a 'cow', so Bill insults Jane twice in one idiom by calling her 'A cow in a china shop.'

Why should you learn idioms?
Because you will not understand English until you do. There are thousands of idioms in English, from one word idioms to entire sentences. Most English speakers do not even realise when they use idioms. How many idioms are there is this declaration?

'I've had it up to here with battling with idioms. The language is awash with them, and you have to wade through a dictionary to get to grips with a simple sentence. Give it a rest!'

The answer is six. And this statement is not even unusually rich in idiomatic language. Some British sports commentators seem to speak only in idioms. While learning idioms is difficult because of their non-literal meaning, it will give you a much richer understanding of the English language and culture

When should you use idioms?
When you are sure that you are using them correctly. Idioms, like slang and swear words, are among the hardest parts of a language because your use must be exactly correct. You may use an idiom that is not right for the situation, and if you use an idiom wrongly, it sounds rather strange. You may use an idiom that means something different to your intended use.
For example:
'To pass' means not to accept an offer.
'To pass out' is to become unconscious
'To make a pass' can get your face slapped it you do it to the wrong person!

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. There are also many books that give you lists of idioms and these are useful if you discover an idiom and want to know what it means. Such a book could be a treasure trove (full of value) or a lemon (a disapointing failure.) And of course you can look for sites like this one on the internet.

So lets roll up our sleeves (get ready to start work), then get our noses to the grindstone (start working hard) and learn idioms until the cows come home (for a long time).

Let's go!