In the middlegame, two minor pieces are often stronger than a rook and two pawns. In the endgame, however, a rook and even a single passed pawn are sometimes better than the two pieces. Artur Yusupov, Build Up Your Chess - 1: The Fundamentals
The best squares for the knight are, basically those in the central zone of the board - d5 and e5 (d4 and e4 for Black). Hans Kmoch, Pawn Power In Chess
Very often the first error is followed by a chain reaction of mistakes, ending in defeat. Therefore it is especially important to maintain your composure and clarity of thought at that very moment when the game begins to go downhill. Alexey Suetin, Three Steps To Chess Mastery
Once you gain the initiative, you must do everything possible to retain it. Follow up actively and aggressively - don't let up! Every move should have a threat. Sunil Weeramantry, Best Lessons Of A Chess Coach
Always look for changes in the pawn-structure that happen in a flurry of tactics - a sure indication that the position needs to be re-evaluated. Alex Yermolinsky, The Road To Chess Improvement
In the middlegame a bishop or knight can easily be a match for a rook, and is sometimes superior. This is particularly true if there are few open lines along which the rooks can operate. Robin Smith, Modern Chess Analysis