What are the things that a 2000 player knows that a 1800 player doesn't know? I'm stuck at 1600-1800 in chess.com. My rating fluctuates within that range. There must be something that I still don't understand that is already understood by a 1900-2000 player.

So I am asking for tips. How can I reach elo 2000? Should I focus on opening?

UPDATE
Thank you so much for the contributors. All answers are great and I really appreciate your efforts for sharing your ideas.
Should I focus on opening?


Do you regularly fail to get out of the opening? Regularly get beaten whilst still in the opening? If yes then you definitely need to work on your openings. 

Do you usually come out of the opening with a playable position? If yes then you are wasting your time spending more time working on openings if your goal is to improve.

Do you get a lot of pleasure working on openings? I know I do and that's why I still waste time on them even though I know my time would be better spent on other things. 

Once you get to something like 2300 then openings start to become a lot more important. At that level and above games are much more often won and lost on superior opening preparation, often geared towards a particular opponent and his known opening repertoire.

Twice in the last 15 years I've prepared a specific opening for a specific opponent and they've gone along and played what I wanted / prepared. In both cases the resulting position was what the book told me was a much better position for me. In one game it certainly looked that way because even though I was playing somebody about 100 points stronger than me I went on to win. 

In the second case I again reached a position which was supposed to be better for me but I couldn't really see what to do and went on and lost. When I put the second game through the computer it didn't think much of the "much better position for me". So, you can't always trust the opening books. The top players, of course, are writing those books. Are they still playing? Are they really going to give away their best moves? Or save them for the next time they play Kramnik or Carlsen?

Bottom line: the effort v reward equation isn't very good. What I've been doing recently on openings is just making sure I have a repertoire which gets me safely to the middle game where the real fight at my / our level begins.

Here's another question for you. Can you mate with KNB v K? If not then I would strongly recommend that you study this endgame until you can win 100% of the time against the computer.

Now I know what you are going to say. "But I've never, ever had that endgame and I'll probably die without ever seeing it in real life" That's true. But the point of learning that particular endgame is that rounding up, corralling and then mating the opponent's king with just a bishop and knight requires remarkable co-ordination between your three pieces. The important thing you gain from mastering this endgame is not the ability to mate with KBN v K but the ability to co-ordinate king, bishops and knights in the endgame and this will come up a lot. 

I remember years ago being very pleased to reach what looked like an even endgame where we each had king, knight, bishop and several pawns against a much stronger player. I offered a draw, as you do. He was very polite and didn't burst out laughing. After he'd beaten me he explained. "Look", he said, "No disrespect, but you are a much weaker player. This kind of position is actually quite difficult. Of course it should be a draw but I know how to play it and you probably don't. So, of course I played on."

Now, I could have just given you the standard reply of "Study endgames" but that wouldn't have really helped because it is too general. 

Now, go away and learn how to mate with KBN v K. Practice until you can beat the computer 100% of the time and you will get a lot more points. Not only will you start drawing games you would previously have lost and winning games you would have let slip away to a draw but you will also recognize earlier in the game how you can steer the game to the kinds of positions you know how to play well in the ending and avoid ones which are not so good.