Something I don't understand happened in move four of London opening.

4...Nc6?! by black is marked as an inaccuracy and loses like one pawn worth of engine points

This gives the position below, where the engine really wants me to pin that knight to their king. Instead of doing that, I went bishop to d3 and it's marked as an inaccuracy.

Somewhere here, black made a mistake worth a pawn and I don't understand why what they did was bad and I don't understand how to play white moves to get an advantage from that inaccuracy.

I know that against the london, black is not suppose to put the knight in front his c-pawn. He is suppose to push the c-pawn 2 squares and put the knight behind, or put it on d7. I think that's because it limit black's pawns play and the knight doesn't have good prospect.

That's for why it's bad. As for how to take advantage, I'm not really sure. I don't think you have anything specific to do. Black is having a harder time playing against you so the engine gives you one pawn worth of advantage IMO.

Yes, typically in a London or Queen’s Gambit you want to push the c-pawn as black, so to challenge white’s d4 pawn.

Black wants to play 4...c5 so as to attack pawn d4 and also to play 5...Qb6 attacking pawn b2 left undefended after the early 2 Bf4.
After 4...Nc6 indeed 5 Bb5 gains total control over central square e5: allows 6 Ne5.

i don't want to insult you
but it's hard understanding opening principles and therefor why nc6 is bad before pushing the c pawn, when you only play the london system

@AKilburg and @J3742 those abstract ideas were about as far as I had understood the position. They don't explain some of the concrete things I mentioned in my post, like why that pin follow-up is so uniquely good or why putting my light bishop to the normal position loses all of the advantage.

Thank you @tpr I looked at the next few computer moves and indeed Ne5 ends up happening several moves later eventually doubling some of black's pawns so maybe you are right about that move.

@jes thank you for your comment, but I'm surprised you didn't also remark on my low rating! Surely it's also difficult for someone with my low rating to understand opening principles as well compared to your relatively higher rating.

Nc6 is playable if f6 is possible

@Mahith1708 what is the connection between Nc6 and f6?

F6 preparing e5 and nc6 does the same

@chessfan124 you are totally right, I miss that part of your comment, sorry about that