Many students ask me about idioms: What are they? Should you use them? Do they help your score?
My dictionary defines an idiom as "a group of words whose meaning is not deducible from the meaning of each individual word". This means that you cannot understand an idiom by analysing it word for word. For example, "it's a piece of cake" means "it's easy". Phrasal verbs are also idiomatic expressions (e.g. "look up" can mean "search in a dictionary").
English speakers use idioms all the time in conversation, but less so in formal/academic speaking and writing contexts. However, we often write things like "focus on an issue" or "the key to solving a problem" and here we are using 'focus' and 'key' in a figurative or idiomatic way.
Idioms in IELTS
You need to be really careful when using idioms in your IELTS test. Please don't learn lists of idioms; if you use them in the wrong way, your speech/writing will seem forced and unnatural. Also, remember that many idioms are informal or clichéd. So, what should you do? Read my tip below.
You can only be sure that you are using an idiom correctly if you have seen or heard it used in context. For example, if you've read about someone who "set up a business", you can use that phrase with confidence (and it might help your score). If you've only seen the idiom on a list, don't use it.