When one of my students tells me they want to improve their vocabulary -- really the first thing that I say is to work on phrasal verbs and idioms---. This doesn't always go over so well. When a non native English speaker learns English, they learn it in a clean and pristine environment, possibly by someone who speaks the King's English (a Brit). In the US, we sprinkle phrasal verbs (expressions like iron out, figure out, make out, put up) in liberally with our English. When one of my students asked me to explain a phrasal verb, I did.... but using another phrasal verb, which led to another phrasal verb. Of course I didn't even realize it. My English is so infused with phrasal verbs that often I don't know it. Okay... let's talk about one specific phrasal verb. The phrasal verb "iron out" literally means to remove some unevenness, such as a wrinkle or crease, from cloth by ironing the creases out. The phrasal verb means to remove some obstacle or difficulty by solving the problem or compromising. If someone asked me what "iron out" meant I might possibly say -- to figure a problem out, (figure out is a phrasal verb that means to solve something), or I might say we'll "work on it" which is yet another phrasal verb meaning that we'll solve it (the problem).
So, to really master English - get acquainted with phrasal verbs. Americans use these liberally; the literal meaning of the phrasal verb does not help you to figure it out (ok... see I used another one!)