Writing for both Academic and General Training requires you to write a short essay in an academic style. Online IELTS Coaching in Laxmi Nagar. Just like formal letters or academic reports, it is not appropriate to use informal language, such as idioms, when writing this style of essay. Idioms can be used in the Speaking test and can help you get a higher score because you are using a more informal, natural and conversational type of language, but this does not mean that you should use them in every single sentence.