General Principles: Openings Pt. I

The main aim of the opening is to get all your pieces into working order - i.e. developed. The moves of the minor pieces and the Queen have a double purpose:

1. to get those pieces into play themselves and
2. to clear the back line for the rooks.

Development is complete when the rooks are connected on the back rank and at least one of them is on an effective file, both if there two effective files. (pg. 44)

The second aim of the opening is to get a good share of the center squares.

Where you don't play e4 (...e5) early, never block your c-pawn. (pg. 45)

Don't finachetto a bishop if an avenue is already opened to it. (pg. 45)

Do not let a bishop be shut in by a one-step move of the e-pawn or d-pawn. (pg. 46)

To see how you stand in development see how many moves each side needs before their rooks will be connected, with one rook on an open or half-open file. Add on half a move (or "tempo") for the player whose move it is, and take the diference. If one player is 2 1/2 tempos ahead and has a good share of the center as his opponent, then that player usually has a winning advantage (advantage of a pawn plus). (pg. 46)
An exchange loses a move(a "tempo") if the opponent recaptures with a developing move. (pg. 47)

To avoid getting behind in development refrain from playing any non-developing move, unless you can compel the enemy to make a non-developing move to counterbalance yours.  (pg. 46)

Consider taking a center pawn if the pawn is threatening to take your pawn or to advance and hit a piece. (pg. 48)