1. Piece of cake – Very easy.

2. Costs an arm and a leg – Very expensive

3. Break a leg – good luck!

4. Hit the books – go and study

5. Let the cat out of the bag – to expose a secret .

6. Hit the nail on the head –  doing or saying something that is precisely right.

7. When pigs fly – something will never happen.

8. You can’t judge a book by its cover – never judge  appearances.

9. Bite off more than you can chew –  to attempt to take on a task that is too much for you to handle.

10. Scratch someone’s back – to help someone out with the assumption that they will return the favor in the future!

11. It's raining cats and dogs - when it rains heavily

Note :

Dear readers , the list of course is not exhaustive , you can suggest other idioms . Let's make the list as long as possible .