In playing this Pawn your object is afterwards to play Queen's Pawn to
Queen's 4th square, and thus establish your Pawns in the centre; but
Black foresees the intention, and thinks to prevent its execution by
bringing another Piece to bear upon the square.

4. Q's Pawn to Q's 4th. 4. Pawn takes Q's Pawn.
5. Q's B's Pawn takes Pawn. 5. K's B. takes Pawn.

Here you have played without due consideration. Black's third move of
Queen's Knight to Bishop's 3d square was a bad one, and afforded you an
opportunity of gaining a striking advantage, but omitting this, you have
enabled him to gain a valuable Pawn for nothing. Observe, now, your
reply to his third move was good enough, (4. Queen's Pawn to Queen's 4th
square), but when he took your Pawn with his, instead of taking again,
you ought to have taken his _King's Bishop's Pawn_ with your Bishop,
giving check: the game would then most probably have gone on thus:--

5. K's B. takes K. B. Pawn (ch.) 5. K. takes Bishop.
6. Queen to K. R's 5th (check). 6. K. to his B's square.
7. Queen takes K's Bishop (check).

In this variation, you see Black has lost his King's Bishop's Pawn, and
what is worse, _has lost his privilege of castling_, by being forced to
move his King; and although for a moment he had gained a Bishop for a
Pawn, it was quite clear that he must lose a Bishop in return by the
check of the adverse Queen at King's Rook's 5th square. It is true that
he need not have taken the Bishop, but still his King must have moved,
and White could then have taken the King's Knight with his Bishop,
having always the better position.