I'm 1400 as well.
Endgames. I wish I was good at endgames. Often, the engine tells me after the game that I have played really well throughout most of the game and then if my opponent has the sense to simplify the position down to an endgame, It tells me that I have made very questionable decisions.
I'm also particularly weak when it comes to defending against passed pawns.
I've been meaning to study the games of Jose Raul Capablanca for ages now but I never get around to doing it. It might not mean much coming from me but I think attempting to understand his endgame capabilities would be a significant advancement.
Good luck.
Hi, I'm 1400 in rapid right now. I only know basic openings and do puzzles most of the time other than games. What should I study to improve myself? Does anyone want to learn chess together?