Please help me...

Nxe4 is not bad, it is just not ideal. g3 first would have been even stronger

21..Nxe4 (K is reserved for the king) trades a central pawn for a flank pawn on the kingside so it might look good, but it breaks two principles I try to follow when playing the KID:
- you should keep your space advantage on the kingside and force your opponent to attack on the queenside: as long as your pawns stay well protected on f4 and g4, you will have space to improve your pieces and your opponent will either have to shuffle his pieces or form a plan on the queenside.
- if possible you should keep your light square bishop for a potential sacrifice on h3: that might only apply when the g file is the semi-open one, so I'm not too sure about this one ;)

IMO, it's the remaining pawns. By playing g3 first, you remove a pawn covering white's king. By playing Nxe4, you allow Bxg4 which gives white a protected passed pawn.

Yes first 21...g3 and then 22...Nxe4 is stronger than 21...Nxe4. 21...g3 breaks up the pawn protection of the white king and opens the h-file for your rooks and queen. 21...Nxe4 allows him to trade his bad bishop Be2 on the color of his pawns for your good bishop Bc8 not on the color of his pawns..

if order: g3, hxg3, fxg3, Bxg3, Nxe4
spicy idk just one variation i see

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