Strategy & improvement
Most rating points below 1400 are won and lost on a few recurring mistakes. Fix these and you'll climb fast.
Leaving a piece undefended where it can be taken for free. Fix: before every move, check whether your move leaves anything en prise (capturable).
Playing your own plan while your opponent is about to take something or mate you. Fix: ask what does my opponent want? every move.
It gets chased and you lose tempo. Develop knights and bishops first.
A king stuck in the centre gets mated. Castle within the first ~10 moves.
Blitzing out moves causes blunders. Use your time; do a quick safety check before committing.
Learn principles (centre, development, king safety) instead of move lists.
Trading helps the side that is ahead in material or has the more active position. Don't trade just because you can.
Attacking with one or two pieces before developing rarely works. Get everyone in the game first.
Many beginners study openings and never endgames — yet endgames decide close games. Learn basic checkmates and king-and-pawn play.
You repeat mistakes you never notice. After each game, find the move that lost it.
Start fixing them now — play a free game and review it after.
Hanging pieces — leaving them undefended where they can be captured for free. A simple per-move safety check (is anything of mine attacked and undefended?) prevents most lost games.
Slow down and run a checks/captures/threats scan before each move, looking at your opponent's options as well as your own. Reviewing your games afterward makes the pattern stick.