what is the idiomatic expressions
Idiomatic expressions or idioms are phrases that often have two, sometimes three meanings - a literal meaning and one or two idiomatic meanings. The idiomatic meanings often have no obvious connection to the actual words.
She is over the moon means
1) She is very happy or excited (idiomatic meaning)
2) She is in a space shuttle over the moon (literal meaning)
He has bitten off more than he can chew means
1. He has taken on too much work or responsibility and cannot handle it (idiomatic meaning)
2. He took a big bite of some food item and has too much in his mouth.
Every language has hundreds, maybe thousands, of idioms (idioms in English even vary according to region!) and they have to be learned just like any other vocabulary item!!
Hope that helps