The real purpose behind "knights before bishops" is "keep the widest choice" or, as it is generally expressed, "reserve the greater option." Cecil Purdy, Action Chess: Purdy's 24 Hours Opening Repertoire
The best time to study an opening is immediately before and/or immediately after a game, because then the stimulus to learn is at a peak. Leonard Barden & Tim Harding, The Batsford Guide To Chess Openings
The traditional 'weak points' f2 and f7 are especially vulnerable in 1.e4 e5 openings. It pays to watch out for tactics, including sacrifices, on these squares. John Emms, Starting Out: The Scotch Game
Doing tactics and calculation has to be my key piece of advice for the club player. These sorts of exercises will just make you quicker at calculating and over time you'll see more. Shreyas Royal, Chess magazine
Don't assume a move you hadn't expected is bad; often it's just a sign that you should look at the position more closely to see what else you haven't seen. Jonathan Rowson, The Seven Deadly Chess Sins
Some gambits are of dubious or marginal worth if the opponent knows how to defend precisely. But others are perfectly sound, and your chess education will be seriously lacking without exposure to this unique opening form. John Watson, Mastering The Chess Openings - Volume 4