I lost to a ~700

I played my friend who is 400 rated on Chess.com. We played four games and this is the first; I won the next three rather easily. Any tips? This is gonna haunt me for a while...

You developed your knight a bit too late, and your friend gained the advantage because of it

Also remember that every move the opponent plays has an intent, whether to defend stuff or to prepare an attack

15 Qxd5 (also 15 Rxe4+)

and there was the simplifying 16 Qxd5 Qxd5 17 Nf6 ++

But your continuation was fine too. Your only problem was that you fell into that cheapo mate at the end. So next time play 20 Qxe6+ (once queens come off, Black has a lot less chance of a swindle). Or 21 Qe2. Or for that matter consider 21 Be3--since you're a piece up and Black has a rather gaping kingside, you don't even have to worry about the hanging b-pawn (inasmuch as Black's capture of it will allow you to deploy your bishop to a great square): Qxb2 22 Bd4 Qc2 (forced to prevent 23 Rxg6+ hg 24 Qxg6#) 23 Re7 Rf7 24 Rxf7 Kxf7 25 Qd7+ Kf8 26 Bg7+ Kg8 27 Bh6, when Black is hard-pressed to guard against the twin threats of 28 Qg7# and 28 Qxd5+ followed by 29 Qxa8#.

Yeah, I definitely had some more sensible moves on move 15; I should’ve seen Rxe4+, but wow is Qxd5 tricky—double check if taken! Of course the checkmate was the biggest issue, so simplifying was easily my best option. In hindsight, I think I am afraid of rook endgames and need to study them more. I was afraid of a position where I could have easily converted with the right play and took the more conciliated route—to devastating effects.

@AetherMaster101Chess said ^

I played my friend who is 400 rated on Chess.com. We played four games and this is the first; I won the next three rather easily. Any tips? This is gonna haunt me for a while...

https://lichess.org/RbBjdl0jSbcN

on the third move Bb5 is the Roy lowpess and one of the top moves

@AetherMaster101Chess said ^

Yeah, I definitely had some more sensible moves on move 15; I should’ve seen Rxe4+, but wow is Qxd5 tricky—double check if taken! Of course the checkmate was the biggest issue, so simplifying was easily my best option. In hindsight, I think I am afraid of rook endgames and need to study them more. I was afraid of a position where I could have easily converted with the right play and took the more ...

qxd5 is a natural move, i think for a 1700 u should see it easily, but that was not the main problem in the game. the move u played is also good. i think u should work more on tactics, the whole game u had a lot of winning chances but u messed up because of tactics. improve ur calculation.

on move 8 when u pin his knight, u have to realize the only way he can defend it is with f5. nc3 f5 bd5 and ur already winning. in a game when their queen and king and knight are in the same file u should immediately know something is off and u have a winning position

@rbhoopathi04 said ^

Yeah, I definitely had some more sensible moves on move 15; I should’ve seen Rxe4+, but wow is Qxd5 tricky—double check if taken! Of course the checkmate was the biggest issue, so simplifying was easily my best option. In hindsight, I think I am afraid of rook endgames and need to study them more. I was afraid of a position where I co...

For whatever reason I thought d5 immediately was an equal position. Obviously not.

Instead of 9.Bd3?! the very natural 9.Nc3 would have developped a piece while creating a threat on Ne4. Or if you absolutely want to move your bishop twice or even thrice in the opening ;) when you see that the only valuable Black's answer to maintain the attacked knight is d5, then Bd5 seems a better choice than Bd3.

Instead of 15.Nxe4, 15.Qxd5 was directly winning another piece.

Instead of 16.Nxc5+: you are already a piece up. Instead of capturing a pawn, you should have considered the double checks 16.Nf6+ or 16.Nd6+, which definitively prevent Black to castle.

Instead of 20.Rxe6: 20.Qxe6+! wins the pawn and forces the exchange of queens. Black is left a bishop down, virtually no counterplay and a lost position. It was safer than looking for a quick checkmate.

Hence my only piece of advice concerns your time management. What strikes me is the time spent on your moves: 2 minutes 30 for the first 20 moves, i.e an average of 7.5 seconds per move. That is a near-blitz speed. In a 10-minute game, you should play at half that speed. After the awful 8...Qe7?? it seems there was an immediately winning tactic somewhere. You had a lot of time left (around 9 minutes) and could calculate a bit more to find a better move than 9.Bd3.