Idioms are conventional expressions such as  "get on somebody's good side".  Proverbs express thoughts and are usually complete sentences. There is a book called Proverbs in the Bible filled with them.

There's nothing wrong with using simple language. Even simple language is full of pitfalls and subtleties. I habitually check everything even if I have the slightest doubt as to whether some aspect of what I've written is grammatically correct. That's quite anal but my skills would suck had I not taken up that habit. It's so easy to do when you have Google, dictionaries and discussion forums about grammar at your fingertips.  Another method that I haven't really used that much is so far is looking up words I don't know for things I deal with when I go about my daily business. For example, how many learners of English would know what <i>bensatankin korkki</i> or <i>bensatankin luukku</i> are in English? You rarely need to talk about those things until you do. I once had a problem with a rental VW Golf whose fuel door I could not open. In nearly every other car I've ever driven there's a lever near the floor to the left of the driver's seat you pull up to open the fuel door it but no such lever could be found in the Golf. I almost had to call the car rental to ask but I got it by reading the manual.