Development of Pieces

First and foremost in the principles of good play in the opening is quick development of pieces. Unnecessary delay like capture of insignificant pawn should be avoided as this will give the opponent time to speed up development and seize the initiative.
In the following game White failed to foresee the danger his king may be in when capturing a pawn. That single loss of time was crucial, enough for Black to disorganize the position of White's pieces and to start a strong attack.