An advantage in mobility must be exploited energetically (usually by an assault), otherwise it will be dissipated in short order. Reuben Fine, The Ideas Behind The Chess Openings
The probability of a blunder increases in quiet positions when a player has difficulty in switching from general considerations to a task of calculation that suddenly presents itself. Vladimir Popov, Chess Lessons
The mere occupation of the centre by pawns does not spell control of the centre. So long as the opponent has centre or wing pawns yet to be heard from, centre-control remains in abeyance. Al Horowitz & Geoffrey Mott-Smith, Point Count Chess
Statistics culled from numerous tournaments reveal that rook endings comprise 50% of all endgames, and it is for this reason that a study of them is of great practical value. Grigory Levenfish & Vasily Smyslov, Rook Endings
Learn to use a chess program, eg ChessBase, in order to improve speed and efficiency (when studying the opening), and to make study more fun. Andreas Tzermiadianos, How To Beat The French Defence
At master level (approximately 2360 elo) and above, a pawn deficit is generally decisive. From there down to 1800 elo a pawn deficit on its own need not be disastrous, unless the inferior side cannot avoid a simple losing endgame. Below that level, a two-pawn deficit (or a piece for a pawn or two) can often be overcome. David LeMoir, How To Be Lucky In Chess