Where we can use Idioms
You can use idioms anywhere, anytime, Karun, though you might restrict their use in a formal context, such as an essay or a speech. Native speakers never think, 'Ah! I am about to use an idiom!' Idioms are a reality that only teachers and students ever really consider; and they DO consider them because they have, in comparison to 'normal' phrases and sentences, an odd nature.
Mark in Rocky Gully