The Ruy Lopez Opening, also known as the Spanish Game, is one of my favorite chess openings. It is a popular opening at all levels. The Ruy Lopez starts with the moves 1.e4 e5 2. Nf3 Nc6 3.Bb5.
The bishop move puts immediate pressure on Black's center by threatening the knight on c6. White often maintains long term pressure on Black with this opening, is ready to castle immediately. White usually plays c3 and d4 to strike in the center. This opening has been played for many hundreds of years and so there is a lot of theory. There are many popular responses for Black, and I will list a few here:
The Morphy Defense, 3...a6
The Morphy Defense is one of the most common responses by the Black player. It threatens the bishop on b5 and forces white to make a decision about what to do with the bishop. White usually moves 4.Ba4 here, maintaining the pressure on the knight while not giving Black the advantage of the two bishops. This usually continues with 4... Nf6 5.0-0, after which Black can play 5...Nxe4, the open game, where Black plays for counterplay, or 5...Be7, the closed line, where Black seeks to maintain the pawn on e5. The diagram below gives a common continuation for the closed line.
White can also play the exchange variation, 4.Bxc6. White releases the tension in the center and doubles Black's pawns, but Black gets the advantage of the two bishops. This variation generally (but not always) leads to a simpler game. After Black plays 4... dxc6, White should not play 5. Nxe5, after which 5...Qd4 wins the pawn back, but simply castling is best. See the diagram for an explanation.
The Berlin Defense, 3...Nf6
The Berlin Defense is a very solid response to the Ruy Lopez. I don't know too much about it, so feel free to tell me things I missed. The move leaves the White bishop on b5 where it can be attacked later. It often leads to a queenless middlegame and Hikaru seems to like it, so it must be good.
The Classical Defense, 3...Bc5
The Classical Defense is another good response to the Ruy Lopez. Black develops his dark-squared Bishop to c5, putting pressure on f2 and temporarily restraining d4. White usually responds with 4.c3 or 4.0-0, and the game will open up quickly, though white could also try the fork trick 4.Nxe5 Nxe5 5.d4. Black will develop his knight to f6 later and prepare to castle. This is a good response for beginners as Black develops quickly and gets good piece activity.
3…d6, the Old Steinitz Defense.
The Old Steinitz Defense uses the d-pawn to support the e-pawn and allows the light-squared bishop to enter the game, at the cost of a slightly cramped game. Black will usually break the pin with a later ...Bd7 or possibly give a pin with ...Bg4. This can be more passive so I recommend the Classical Defense or the Berlin Defense for people who want a more open position.
3...f5!?, The Jaenisch Gambit
The Jaenisch Gambit is an aggressive gambit line to attack the Ruy Lopez, trying to undermine white's center from the side. It opens the f-file for a Black rook and often leads to very complicated play. Often, White has an extra pawn or two while black has the initiative. Black's king can become exposed however.
There are a few other moves that Black might try, such as Nge7, Qf6, g6, or Bb4, but in general playing c3 and d4 to open up the game and control the center is something you should do, and castling quickly is always a good idea too.
If I missed something, please tell me, and of course, feel free to add your own ideas as well. You can also share games you have played with either side of the Ruy Lopez; please annotate them if you do. I will probably share a few of my games as well.