The most common response to 2.Nf3 is 2...Nc6. This move guards the Pawn on e5 and, at the same time, attacks the d4-square.
Now if White insists on playing 3.d4, the game will probably continue 3...exd4 4.Nxd4, reaching the following position.
Here Black gets a playable game with either 4...Bc5 or 4...Nf6, attacking one of White's central outposts. There is nothing wrong with White's position and this opening has its adherents, including some of the world's best players. Other good players judge that White has squandered part of the first-move advantage and seek more.
The most popular alternative to 3.d4 is 3.Bb5. This Bishop move, by attacking the Black Knight, defender of d4 and e5, threatens 4.Bxc6, followed by 5.d4 or 5.Nxe5. In fact, neither threat is considered dangerous by experts in this opening, leaving Black with many responses to 3.Bb5.
The most popular response is 3...a6. This opening has been played hundreds of thousands of times, by some of the greatest players that the game has seen. In spite of so much practical experience, we still don't know if 3...a6 is the best move in the position and we may never know.
