I play with white last knight on FICS and after 1.e4 e5 2.Nf3 Nc6 3.Bb5 black played 3... Nc4!?.

I then played 4.Nxd4 exd4 5.d3

But I was unable to prevent any black threat later on.

The game continued with 5...c6 6.Ba4 Nf6 7.c3 Bc5 8.cxd4 Bxd4 9.Nc3 0-0 10.0-0 d5 11.Re1 Ng4!



with a clear adventage for Black and I eventually lost the game.

My question is: What is the best response for 3... Nc4??

I was thinking al lthe afternoon in this line, and I came up with this variation:
1.e4 e5 2.Nf3 Nc6 3.Bb5 Nc4 4 Ba4




The game then may continue as follows:

4... Nxf3 (the only reasonable way of defending the e5 pawn) 5. Qxf3 Nf6 6. c3 Bd6 7. d4 0-0 8. 0-0

And white dominates the center.

I don't know much about this line, but have you checked to see how grandmasters play this? I think this variation is called Bird's Defense. Head to chesslab.com or some other database and plug the first three moves in, and look to see how the grandmasters play against this. That should give you some ideas.

--Fromper

I looked at Bird's defense. The line is as follow:

1.e4 e5 2.Nf3 Nc6 3. Bb5 Nd4 4. Nxd4 exd4 5.0-0 Bc5 6.d3 Ne7 7.Qh5 Bb6 8.Bg5


Also there are three variations: Black can play either
5... Ne7 (Paulsen variation)

5... g6 leading to 6.d3 Bg7 7.Nd2 Ne7 8.f4


or

5... c6 leading to 6.Ba4 Nf6 7.d3 d5 8.Bg5 dxe4 9.dxe4 Be7 10.e5 Nd5 11.Bxe7 Nxe7 12.Be3 0-0 13.Nd2


This is what Chessmaster 10 told me, but it's all theory.

I guess I can go and check the games at chesslab.com and see how GMs play this line but I'm not a GM and I do not play against GM (I'm a class C player), so is not probably that any move that any GM plays as Black will be played against me in a match.

And in top of all this: I just hate to have an enemy pawn in my space so early in the game.

6.Bc4 was better giving white a slight edge.

7.c3 is a bad move which trades off an advanced and potentially weak pawn which was blocking the scope of black's dark squared bishop. You justified his somewhat antipositional play by trading off his only weakness and weakening your own position. 7.0-0 seems to lead to a slight edge for white.

Trading this pawn is good for black.

One of my mentors used to play this line as black in master level play when he first arrived in North America. He was not playing it by the time I met him.
It should lead to a slight edge for white.

Your seventh move was the problem.

Crash

The line as mentioned above is the Bird's Defense.
Fine called the Defense unsound. Although in the 30's the Soviet players
tried new ideas (at the time) to revive the line.
Going to recent games, ECO 61 (Bird's)
4. Nd4 (Also recommended by Fine in PCO) ed4 5. 0-0 Bc5 (Fine in PCO gave
Ne7! as best, and a try g6) Keres and Geller in analysis looked at c6.
In any event White got the edge. See Chesslive.de for recent developments.
An older game was Dr. Olland-Alekhine (Scheveningen 1923) where White
tried a recommendation of Dr. E. Lasker (4. Ba4 Bc5).
Blackburne seemed to have tried the Bird against Capablanca, and Alekhine
at St. Petersburg, 1914 which not good results.