What was my first error?

If we JUST examine the tactic ONLY, then 6. ... b6 is the error that sets up the loss of material.

Nc6 ? black should try Nbd7 c6 or c5 Nc6 b6 was definetely a blunder

Can you say something about how I can know Nc6 is a blunder at the time that I'm making it? To me, without the benefit of hindsight, it appears to be a perfectly innoccuous and normal opening move. Developing a piece... control of the center... etc.

What is it about Nc6 that springs out at a better player making it clearly an inferior move?

KrisRhodes:

6... Be7 7. Bb5 O-O 8. O-O Qd7

I don't think 3...Nc6 is a blunder I just think c5 is better.

Nc6 < e6 < Bf5 < c5

The move ... c5 attacks white's center. The move ... Nc6 prevents you from making this move. Nc6 actually does nothing to control the center because white is in full control of d4 and e5. So even though your knight "attacks" these squares, this will never come to anything.

Okay, thanks so far.

I guess I don't have the hang of "gambits." When I look at c5, all I can think is, "well, he'll just take the pawn, and now I'm down a pawn."

Certainly now he's got doubled pawns, but how can I help myself see that this is a real advantage, especially in light of the lost pawn?

3. ... c5 4. dxc5 Qa5+ followed by Qxc5 and you aren't down a pawn at all.

Or 3. ... c5 4. dxc5 e6 and you are threatening Bxc5. The compensation for allowing him to take on c5 isn't "doubled pawns" it's removing one of this center pawns and forcing him to play non developing moves on the wing if he wants to hang on to the pawn on c5.

In any case, 3. ... Nc6 is pointless because the knight will always lose the battle for d4 and e5 if you allow white to keep the pawn structure he has. That knight is better on d7 -- it supports the c5 push and can go to f6 after the f6 knight goes to the hole on e4.

U SHOULD ALWAYS TRY AND ACTIVATE UR POSITION. IT IS NATURAL IN THIS OPENING TO PLAY A C5 BREAK . nC6 IS JUST PASSIVE. USEEMED TO BE UNDONE PY IDEAS OF THIS OPENING

So I don't consider moves like c5 because they require me to move two pawns in the opening--which I am told in other contexts is a no-no.

What is it about c5 that makes it okay, in this case, to move a second pawn?

I guess, on review, that question's already been answered in the thread.

So--say I do c5 instead of Nc6. He takes the pawn. Should I then move [i]another[/i] pawn (the d pawn) to make way for an attack on the c pawn with my bishop?

Ne7 looks okay, though it puts the knight in the way of my other bishop--requiring yet another pawn move in the opening to get the bishop onto g7.

Are these good thoughts?

Speaking as a raw beginner in terms of chess "knowledge" and someone who probably would be ranked in the 1300s or 1400s if playing online --  Nc6 all but begs for that pin that got you in trouble.  So, you know, not exactly a high level reason, but still a reason.