I am preparing for the IELTS writing test. Can I use idioms in this test?
For example, "it's not all its cracked up to be" or something like "I bend over backwards to help them, but they try to put me down more often then not".
Idioms are a major (and for non-natives, sometimes confounding) part of English language. A measured use of them would help to demonstrate your English writing skill. However, knowing exactly when and how to use an idiom naturally really can be tricky for a non-native speaker, so when you do decide to use an idiom in your writing, make sure that it fits well the context within which you're using it. If you have learned an idiom but aren't confident about its appropriate usage, leave it out of your writing (IELTS and similar tests are not a good time for experimentation).
Thank you for posting a link to two IELTS sample writing prompts, with graded sample essays. Here is my interpretation of how they are graded.
The IELTS does not encourage writing essays that are short, clear, and to-the-point.
You should not go out of your way to use idioms.
Idioms are an indispensable part of the language so it is not a crime to use them.
As a former IELTS Examiner, I would say that they can both make or break your writing. If you overuse them and put in all wrong places, they will slash your points. If you use a few of them but properly, they will enhance your writing.
So my advice is to stick only to the ones you are 100% confident about.