En passant. It does not take a pro. Just someone who knows the rule.
Easy easy puzzle

Interesting escape.
49. Rxd6 is a game losing blunder but luckily for you Black responded with a blunder of their own Black missed checkmate by not spotting 49. ...Rf3#. But a fairly hopeless position for you anyway due to the Black passed pawn on d4 and the awkward position of the King (how did he end up there?)

To ChessOfPlayer:
Yes, but I was in a stressing game situation, so it took time to realize it. I was tired. Clock is 02:14 AM here in Fin. This is easier when presented as a puzzle I think.

Did you win or lose in the end?
I lost, but I think it was a good game for me. My rating is under 1000 at the moment. Here is a whole match:

7. Qc4 is probably better replaced by Qd1. That way, the queen is not worried about being harrased.
10. Nxb5 was not needed, a3 would have been much better, then you would still have your knight, and just be a pawn ahead.
12. a4 makes the white bishop a liability. Bxd7 would have made the game much easier, as yer pawn structure is much better.
18. Bd2 is much better, lets you keep control of the a file.
36. b6 does not help you, probably, rd1 is better, it wins that passed pawn on d4. He also has no way of defending it. Also, try to trade off the b pawn.
You should develop yer dark square bishop earlier. Also, watch for checks the opponent can do. Remember, the first things to think about are checks and captures.

Ou, thx advices. This I dont understand:
12. a4 makes the white bishop a liability. Bxd7 would have made the game much easier, as yer pawn structure is much better
If Bxd7 then black Rook b8 beat my Queen! Did you mean that?!
EDIT: Ou, sorry! It is chess, you right!

Not bad.
9. Qb3 looks like a big error.
12. Bxd7 looks quite good as stops black castling (otherwise loses the rook)
23. h4 looks quite good, with the idea of 24. g5 hxg5 25. hxg5 26. Rh1 27. e5
But you hung in well

My opinion, in your puzzle is that 1. fxg6, fxg6, 2. hxg6,
Black now has 3 choices.
Kxg6, now Rxd6 helps you. Since now, the knight is hanging, he will be in check and you can get the knight, then return your rook to stop that last pawn. Basically, black loses quickly in this variation.
3. ...Nxg6, Kf5!
This gives your king a change to quickly gobble that backwards d pawn, and give you 2 connected passed pawns. This improves your chances dramatically.
or ignoring the pawn.
Whatever he does, play Kf5, that gives you chances you never had before. If your opponent does not play fxg6, and moves his king, then, you play King to f5 again, basically, in the end, that f pawn actually was the reason for your loss. That is actually why I recommended fxg6. That being said, your opponent missed 47... Rf3#. g5 was actually a blunder. That being said, after Rxd6, Rf3+?? is actually a blunder, as you get to be forced to pick up that e5 knight.
Hope this helps.

To texaspete:
9. Well, I just wanted to threat his pawn b7. I did not figure out nothing else to do.

Qb3 was an ok move, you just need to make sure that you don't get the b5, you did not need to sacrifice your knight to neutralize that threat. I do not like a4, because it does not actually stop that threat. a4 is responded to by b4. a3 though protects b4. and gives your knight and queen escape to a2. Finally, a3 is better, as it is still protected by pawns.

Also, how do you post analysis diagrams?
You mean how I analysing my games afterwards? I do not :(. My browser's javacode does't work. I just play much. Or do you mean how I post games!? Look:

Qb3 was an ok move, you just need to make sure that you don't get the b5, you did not need to sacrifice your knight to neutralize that threat. I do not like a4, because it does not actually stop that threat. a4 is responded to by b4. a3 though protects b4. and gives your knight and queen escape to a2. Finally, a3 is better, as it is still protected by pawns.
Yes, sacrifiyng Knight 10. Nxb5 was stupid, I just did not wanted to go backwards with it.

My opinion, in your puzzle is that 1. fxg6, fxg6, 2. hxg6,
Black now has 3 choices.
Kxg6, now Rxd6 helps you. Since now, the knight is hanging, he will be in check and you can get the knight, then return your rook to stop that last pawn. Basically, black loses quickly in this variation.
3. ...Nxg6, Kf5!
This gives your king a change to quickly gobble that backwards d pawn, and give you 2 connected passed pawns. This improves your chances dramatically.
or ignoring the pawn.
Whatever he does, play Kf5, that gives you chances you never had before. If your opponent does not play fxg6, and moves his king, then, you play King to f5 again, basically, in the end, that f pawn actually was the reason for your loss. That is actually why I recommended fxg6. That being said, your opponent missed 47... Rf3#. g5 was actually a blunder. That being said, after Rxd6, Rf3+?? is actually a blunder, as you get to be forced to pick up that e5 knight.
Hope this helps.
Sorry, hard to follow. I'm not good i n abcdefgh*12345678 notations and your pictures does not be seen in my browser. Maybe I figure out your post some day later.
This is easy for you pros, but I wondered long long time what can I do? I thought it was obvious chessmate and that there is error in code.