Why this opening movement is an error?

Hi

As I am just learning I don't really care about losing games. Actually I prefer, I learn at this moment more by losing and analyzing the game.

But sometimes this is frustrating, for example this black (my side) movement (5. h4 d5) is mark as "error" and I cant understand why. Maybe there are better moves, but why an error?

If pawn takes pawn after that move, then your knight is screwed. You can't move the knight because if you do, bishop will take your queen

Wait. I think I misunderstood your question. What do you mean by h4? In the pic you sent, white didn't move his h pawn. A

No, but you are right. If white now take pawn at e5 I will lost that Knight...

Accidentally pressed the send button. I see now. You meant bisjo

only if opponent wants the knight more than his bishop

If after pawn take pawn he counterattacks with g5, then bishop will go to g3. No matter what he chooses to do with the knight after that, his opponent is now a pawn up

thanks both, very instructive for me (novice)

The simple thing here is the pinned knight. Pinnned peices van’t move, so they don’t defend anything. Therefore, white’s knight wins a free pawn in all variations.