Those who are not native English speakers will understand me. When you learn English, you can be the best at grammar, spelling, etc. BUT there's always something missing. You don't sound as natural as a native regardless of how many hours of English lessons you take. I realized about that specially when I moved to New York. I started hanging out with native English speakers, and even though I could understand and could be understood, sometimes they would use sentences that sounded really random to me. These were expressions!
I remember the first time I heard one of my friends say "it's raining cats and dogs". Can you imagine what I thought?
I consider this part of the English language to be the hardest one to learn. You my understand what "to rain", "cat" and "dog" mean, but the meaning of all together is a bit different. Since those sentences make no sense, it's harder to learn them by heart. However, they make you sound more natural and I think they are what makes a language be enriched. "It's raining cats and dogs" turns out to be my favorite English expression, in fact, I can't wait for the rain so I can use it! (Just kidding).
I'm going to give you a piece of advice. If you hang out with native English speakers, ask them every time you hear an expression you don't understand and then try to use it. That's the way I'm learning some of them. I don't expect to use those phrases as spontaneously as they do, but they make you sound more "English". I think they help you describe things in a way simple adjectives can't. Something like "a picture is worth a thousand words". I must also tell you that sometimes I use expressions in the wrong place, but errors make you learn and they are always fun (if you have nice friends, of course).
What expressions do you usually use? I have been thinking and here are some that I have already integrated in my vocabulary:
I'll play it by ear - I love this one! And no, it doesn't mean I will use my ear to play anything. It means you don't have a specific plan and you will just improvise something.
Break a leg - This one, even though it sounds cruel, is used to wish luck. I find it really funny, and if I translate it to my language, it doesn't have the same meaning at all. But hey, break a leg!
There are thousands of them and they're impossible to learn at once. However, it's nice to slowly integrate some of them into your language to enrich it. I recommend you this website, where you will find plenty of them: http://www.idiomsite.com/. See you soon!