1.Nf3 f6 2.Nd4 Nh6 3.g4

This move prevents 3...Ng4, but now white can't play Qh5 or Qf3.

The main reply for black is 3...c6, 3...b5 is a second line.

If 3...Nc6 then 4.c3 Nxd4 5.e4 and white is better.

If 3...e5 then 4.Nb5 Bb4 5.c3 and white is winning.