To use or not to use — that is the question!
In short, the answer is NOT to use. You need to be really careful when using idioms in your IELTS test. Please don’t learn lists of idioms.
Look through the list of FAQ and the answers on them
Will using quotes, proverbs or idioms in IELTS essay give a higher score ?
Proverbs are certainly not appropriate for an academic writing. Here’s an example of an inappropriate proverb in writing “all that glitters is not gold”. While the meaning might be perfect for your essay, it would be better to write “people should not be deceived by appearances because they can be deceptive”. The latter sentence has a much better variety of appropriate vocabulary for a high score.
Idioms, such as “it cost an arm and a leg” are informal which means they are not suitable for IELTS writing task 2 academic or general training essays. Also, such kind of idioms that many students learn (e.g. «In a nutshell») should also be avoided, because it’s considered to be a cliche.
Regarding quotes, the examiner will give you a score based on your own level of English language, not the words you remember from another person, they will not help your band score for IELTS. It’s much better to use the idea from the quote and write it in your own words.
Is there any idiomatic language which could be appropriate for a high score in IELTS?
Idioms are only one type of idiomatic language. There are other forms more suitable to IELTS writing, for example «the key to solving a problem». That is an acceptable type of idiomatic language for a high score in IELTS.
In addition, some idioms are acceptable if you use good idiomatic phrases without realising it.
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 and it might help your score.
Can quotes, idioms or proverbs be used in IELTS speaking section?
Proverbs and quotes are not usually academic and for that reason are better suited to speaking. However, don’t lose your fluency score because you are trying to remember an idiom. In the IELTS speaking test they can really help boost your score, if you use appropriate, natural language in the right context, however, they can also lower your score, if you don’t use them correctly. So, don’t try to fill your answers with idioms or proverbs. Using quotes is not really suitable for IELTS because they are not your own words.