• Idioms or Idiomatic expressions are colorful expressions such as “to have a bat in the belfry”
• Set phrases are ways of speaking that cannot be translated literally, such as “it’s no use”. When learning a new language, there are a few hundreds of these phrases you just need to learn by heart.
• Proverbs are little bits of wisdom such as “Better late than never”. Many proverbs have been translated in various languages, but some will be unique to the language you study.
• Slang is a set of words used by young people, criminals and when speaking in a low speech register. For instance ‘cops’ or ‘pigs’ for ‘policemen’. It is not a good idea to use these words unless you really understand in what context native speakers use them. It’s OK to hurl a few at your language teacher for fun, but in a real-life context you’ll be likely to antagonize people or pass for a moron. Slang changes all the time. In Dirty Harry, Clint Eastwood speaks about how ‘Swell’ it is to chase ‘Hoods’. If I used these words when speaking with American people nowadays, they will either assume I want to give a 1970s inflection to my speech and wonder why, or, more likely, think I’m just confused with vocabulary.

Madeleine Albright, when negotiating with former Israeli Prime Minister Benjamin Netanyahu, remarked that he spoke ‘idiomatic English’. In her mind, this seems to be the highest stage of fluency once can reach. Of course speaking with no accent is nice, but there is just no substitute for the richness and color idioms bring to the language.

Idioms are used to conjure a vivid share mental image that sums up a situation under discussion. It emphasizes a shared conception of an archetypical situation. This is akin to shared memories amongst relatives or private jokes amongst friends. For instance, you may say that the train was on time, but if you say that the train was dead on time, this is a stronger expression.

You need to learn and use idioms as soon as you can. Idioms are fun, and they are a source of constant amusement as you progress in your language learning. They are an essential component of functional fluency. The more you use them, the closer your interlocutors will feel to you.

Idioms can be very rude. You need to know what speech register they belong to before using them. If a British gentleman is indecisive, you could tell him ‘Fish or cut bait’ but also ‘Shit or get off the pot’. Both are fine idioms of the English language, but they belong to different speech register. Say the wrong one and you’ll get kicked out in no time.