This is a sample game of chess, recorded in standard algebraic chess notation, and accompanied by commentary.
White has the first move, and therefore a significant initiative. White wins 10% more often than Black simply on the power of that initiative. It is important not to squander the opening advantage with a move that does nothing.
This move for Black is good for the same reasons listed above. Note that the two e-file pawns are not threatening to capture each other, because pawns capture diagonally. Instead they merely block each other until something breaks the logjam. (However, a better choice would have been 1... c5, the Sicilian Defense, because it creates a less balanced game and denies White free rein.)
White makes another fine move. White is mobilizing forces by bringing a knight forward into attack position. Because the white knight is attacking black's e pawn, white is maintaining the initiative by forcing black to react. So now black's choice of moves are restricted to moves that defend the pawn or he might lose the pawn. The initiative may appear to offer a slight edge but at the hands of a master it can be deadly.
White could have moved out the queen instead, but that would be dangerous. The queen is too valuable to trade for any other piece (unless you can trade it for the enemy queen or at least the equivalent, such as two bishops and two pawns, three bishops, or a rook, a bishop, and a pawn, for instance,) so it can't do much by itself unless the opponent carelessly leaves pieces unprotected. So your opponent can keep attacking your queen with different pieces. And while you are simply moving your queen around, your opponent will be able to keep developing his pieces. It is better to wait until there are other pieces in the fray which can serve as shock troops for her.
White also could have moved out the king's side bishop.
That would not be a bad idea, but it isn't clear yet whether the bishop wants to be at c4, b5, or possibly even behind the pawns. The knight, on the other hand, usually goes to f3 anyway, because the other squares it could move to are inferior. On h3 it would not attack the center, and on e2 it would be temporarily in the way.
Black makes a worthless move, entering the infamous Damiano Defense. Black needed to defend his pawn with either 2...Nc6 (main line) or 2...d6 (Philidor Defense), or counter-attack White's pawn with 2...Nf6 (Petrov's Defense). If Black felt brave, he could also venture into the murky waters of 2...f5 (Latvian Gambit) or 2...d5 (Elephant Gambit). Either knight move would maintain the balance of the game by contesting the center.
The move 2...f6 appears to defend the threatened e-pawn, but this is an illusion, as the game shall demonstrate. Black has weakened the kingside, allowing attacks on the f7 square, which is protected by nothing other than the king itself. This is called "opening the door," and this must be taken advantage of.
Black has failed to follow the principles here without any compensation. He has affected the center, but the move 2. ...d6 would have done and gained some control in the queen side. Also he has not developed a piece nor has he facilitated the development of a piece. 2. ...Nc6 develops and gains control in the center, 2. ...d6 gains some control on the c8-h3 diagonal and facilitates the development of the light-squared bishop. Black has also weakened his kingside. Any pawn moves in front of a castled king can be useful weaknesses for the opponent. Since pawns cannot go backwards you should be very cautious of any pawn moves on the side you wish to castle on (usually the kingside).
The moves 2...f5 and 2...d5 seem to not adhere to the principles. Why have these moves been brought up? The fact is the best moves in a position will sometimes break principles, but they will offer compensation for whatever gains are lost by not following the principle. So in a sense the principles tell us what questions to ask ourselves when making a move, such as "Should I develop now?" So it is important to understand the principles in order to decide if you are getting enough compensation. It is very bad to blindly do whatever the principles dictate.
White attacks immediately, sacrificing the white knight for two pawns, although generally White would need three pawns for the knight to have material equality. Before making this sacrifice, White has to calculate to make sure he is going to gain something that compensates for the lost material. In this case, as the following moves show, White knows that he will gain an attack on the enemy king, forcing Black to either surrender more material or allow checkmate.
Black makes the only move consistent with 2...f6, but it is not best. The best try for an equal game was 3...Qe7, skewering White's knight, pawn, and king. That is to say, the queen would indirectly be attacking everything in the e-file. After the knight moved away, the queen could take on e4 with check, regaining the pawn. White would, however, have a large advantage because of his lead in development and Black's weakened kingside pawn structure, e.g. 4.Nf3 Qxe4+ 5.Be2, likely followed by 0-0 and Nc3 (attacking the queen).
Retaking the knight now merely invites White's queen to jump into the fray with check. The fact that Black cannot afford to take the knight shows that 2...f6 did not really protect the pawn at all.
This move leaves Black in a dangerous position, because the black king is so exposed. Furthermore, the black rooks, bishops, and queen still have no way to get out. The Black position is no more developed than it was at the start of the game.
Another check. Black now has only one legal move. Black can't interpose anything between the king and queen, and can't take the queen, so the black king must be moved out of check. There is only one square next to Black's king which White's queen is not attacking.
Black moves the King, it's the only available move to play. White's queen is a dangerous attacker. However, because it is too valuable to trade for anything, it can only take undefended pieces. Everything in Black's camp is defended by something, so the queen has done all it can do by itself. It is time to bring in reinforcements.
This is an excellent move to keep the pressure on Black because it develops a piece and gives check while preventing Black from consolidating.
If White played less energetically with 6.Nc3, the advantage would evaporate instantly. Black could answer with 6...Be7, giving the king room to retreat to f8. Once Black gets his king to safety, Black might actually be winning. White has only two pawns for the sacrificed knight, which leaves White at a material disadvantage.
Black makes an excellent defensive move. Never forget that moving the king isn't the only way to get out of check!
Admittedly, Black's d-pawn is a dead duck. It is attacked by White's bishop, queen, and pawn, three times altogether, while it is defended only once, by Black's queen. The sacrifice is worthwhile, though, to open up lines for the queen and bishop so they can help with the defense. Now if White fails to find the best continuation, Black has some chance to counter-attack.
White gives check yet again, which prevents Black from doing anything constructive. Let's review the three ways to get out of check:
Capture the piece giving check. Black could play 7...Qxd5. But White would simply take queen with 8.Qxd5+. With such a huge material disadvantage and an exposed king, Black could resign without feeling like a quitter.
Interpose a piece. Black could play 7...Be6. But that would be inadvisable, because the bishop would be defended only once (by Black's king) and attacked twice (by White's queen and bishop). In fact, White could end the game at once with 8.Qxe6#
Move the king. Alas, the only square which is not under attack by White is g6, even further into the open. It beats the alternatives, though.
Now White must think of a way to continue the attack. White would like to play 8.Qf5+, driving Black's king to h6 where it can be cornered and checkmated, but Black's c8 bishop is guarding the square f5. If Black hadn't interposed with 6...d5, Black would now be subject to a forced checkmate. As it stands White has to be more creative to keep the initiative.
Again White finds a strong continuation. White is threatening to force the Black king to h6 after all with h5+. Also the pawn protects the g5 square which is important in some mating combinations. Finally, there is some chance the rook will be able to join the attack down the h-file.
Black plays a tenacious defense in a precarious situation. White's pawn is blocked from further advances, and the king has a new escape square on h7. Black's position is still precarious, but there is no immediate way for White to force checkmate.
Now let us take a step deeper into chess reasoning. White knows Black is on the run for the moment, but if Black has a chance to regroup, the game is far from over. Three pawns for the sacrificed knight is roughly material equality.
If White brings additional forces forward with 9.Nc3 or 9.d4, the obvious developing moves, Black will harass White's queen with 9...Bd6. That would force White to lose time protecting the queen. Black would gain time to get the black pieces out and get the black king to safety.
White desperately wants a quick kill, but can't see how to get it. White is annoyed that Black's bishop on c8 prevents White from playing 9.Qf5+ and administering the coup de grace. Therefore White asks the question, "What if Black's bishop were not on c8? If only that annoyance were removed, I could do great things."
White finds a forceful continuation that puts Black in dire straits. Black's best bet now is to ignore White's bishop and harass White's queen with 9...Bd6, but then White calmly plays 10.Qa5, maintaining the threat on f5 and forcing Black to lose material. One possible line of play is 9...Bd6, 10.Qa5 Nc6, 11.Bxc6 Rb8. The checkmate has been avoided, but now White has a large material advantage (four extra pawns) and can win slowly and surely with patient developing moves like 12.Nc3.
(Note that after 9...Bd6, 10.Qa5 Black cannot play Bxb7 because of 11.Qf5+, with the same play as in the main variation below.)
As it happens, Black does not understand the danger. Black grabs the bishop for a material advantage of Black's (bishop plus knight versus four pawns) and suffers the consequences.
A weak move, allowing White's attack to break through. (A stronger alternative, although still bad for Black, is 9...Bd6 attacking the Queen and developing the Bishop, but White can reply 10.Qa5 as discussed in the note to White's prior move.) This is Black's critical error. From here on, White has a forced mate.
White continues the attack with a special kind of check, a discovered check. White moves a pawn, but it isn't the pawn which gives check. It is White's bishop, attacking from its home square, which delivers the blow.
Note that Black's king has no legal moves, and White's bishop is safe from capture, so interposition is the only option. But because it is an option, this is not, yet, checkmate.
At this point White has an easy win with 12.Bxg5+ Kg7 13.Bxd8. The material advantage of a queen and five pawns for a bishop and a knight is overwhelming. However, weak players have been known to play on in completely hopeless positions rather than resign. In order to forestall a long, boring mop-up operation, White looks for a direct kill.