I. (3) Qxd4 Kt-c6
          (4) Q-e3

Now P-d5 is not possible because of Pxd5 check.

(4) ...            Kt-f6
(5) Kt-c3         B-e7
(6) B-d2          P-d5

Threatening to win a piece by P-d4.

          (7) Pxd5 Ktxd5

Now White has no center-Pawn either; Black has a perfectly good game.

      II. (3) Kt-f3 Kt-c6

Most players would be tempted to play P-c5 instead.  But this must be bad as it is a Pawn move which is not necessary.  Black does not need to defend the Pawn d4 as he is a Pawn ahead; and the development of the pieces is much more valuable than the advantage of a Pawn.  White would obtain a far superior position with (4) P-c3, Pxc3; (5) Ktxc3, after which Black cannot hope ever to free his game by P-d5.

          (4) Ktxd4 Kt-f6

It would be bad to exchange Knights on d4 because White’s Queen would be brought up into a dominating position from which she could not easily be driven away.

          (5) Kt-c3 B-b4

This attacks again the Pawn e4.  Relieving the pin by (6) B-d2 would be no protection as Black could exchange on c3 and then take the Pawn.  Therefore, White must defend the Pawn with another piece.  The Bishop f1, which has to be developed in any case, offers himself naturally for this purpose, but B-d3 is not possible as long as the Knight d4 has to be kept protected by the Queen.  Hence, it is necessary first to exchange the Knight.

(6) Ktxc6          Pb7xc6
(7) B-d3           P-d5!

Again Black has succeeded in advancing his Queen’s Pawn against the opposing center-Pawn, and White cannot avoid the exchange.  To advance to e5 would be against the principle that no Pawn-move should be made in the opening which is not absolutely necessary.  How easily (8) P-e5 could get White into trouble can be seen from the following variation:  (8) ..., Kt-g4; (9) o-o, o-o; (10) Pf4 ??, B-c5+; (11) K-h1, Q-h4; (12) P-h3, Q-g3; (13) Pxg4, Q-h4 mate.

     III. (3) P-c3

With this move White invites Black to lose time in winning a Pawn or two while the White pieces are rapidly developed.  If Black accepts the Gambit [Footnote:  A Gambit is an opening in which the sacrifice of a Pawn is offered for the sake of a speedy development of the pieces.] the continuation might be.

(3) ...  Pxc3
(4) B-c4 Pxb2
(5) Bxb2

or

(4) ...  Kt-c6
(5) Kt-f3 P-d6
(6) Ktxc3 or Q-b3 or o-o

In all of these cases White has by far the freer game and he remains in the possession of his center-Pawn.  For this reason it is much more advisable for Black to decline the Gambit offered and to hasten his development by immediately advancing his Queen’s Pawn, thus: 

          (3) P-c3 P-d5!
          (4) Pxd5

The advance of the King’s Pawn would be quite out of place, as it would not add anything to White’s development so that Black need not mind the loss of the move either which he sustains from the developing point of view in gaining a Pawn by Pxc3.  The future development of Black is not any longer endangered as he has a Pawn in the center and an outlet for his Queen’s Bishop.