Ten Most Commmonly Used Idioms of English Language and Meanings

1. Piece of cake
-something that is very easy to complete

2. Break a leg
-good luck

3. Let the cat out of the bag
-to accidentally reveal a secret

4. Once in a blue moon
-an event that happens infrequently

5. To kill two birds with one stone
- to solve two problems at once

6. When pigs fly
- something that will never happen

7. To feel under the weather
- to not feel well

8. You can't judge a book by its cover
- to not judge someone or something based solely on appearance

9. Barking up the wrong tree
- looking in the wrong place. Accusing the wrong person

10. Burn the midnight oil
- to work late into the night

References:
http://www.smart-words.org/quotes-sayings/idioms-meaning.html
https://englishlive.ef.com/blog/15-common-english-idioms-and-phrases/
https://voxy.com/blog/2012/02/top-10-most-common-idioms-in-english/