idiomatic expression, it is important or not for some one who is learning a second language?
I agree with Peachey. Begginers first need to be very well acquainted with how a foreign language works so they'd be able to understand and later use idioms properly.
It depends on your level. At lower levels your main focus should be on being understood. You get into idiomatic expressions at advanced level because you already have a body of English knowledge to work from.
It's a huge mistake to try idioms at a beginner level, because your chances of misusing or saying the phrase wrong (and sounding very silly) is extremely high. If you're still making grammar and spelling mistakes, you should avoid idioms altogether.
As an ESL learner I do believe in knowling idioms. I think every day conversations are full of idiomatic and also slang expressions. You may choose not to use idioms in speaking; anyway to understand english-speaking people you need to know lots of idioms. I see it's not easy to memorize them but just you need to put in more time & effort. Wish U the best!