7. Qb3
White moves the queen a second time in the opening. While it's usually not advisable to move the same piece repeatedly in the first few moves, this maneuver creates two strong threats: the b7 and f7 pawns are both under attack, and Black has no way to defend both!
Both of these attacks threaten much more than a pawn, which makes them very dangerous. Losing the f7 pawn could lead to a mating attack; meanwhile, after Qxb7, Black's rook on a8 is hanging. Black must find a way to minimize the potential damage.
7. ... Qe7
The best move in a tough position (although the computer program Rybka 3 also gives the surprising Bc5 as an option). This queen move directly protects f7, while also limiting White's gains again the b7 pawn. Should White play Qxb7, Black can reply with Qb4+, forcing a queen trade.
Unfortunately for Black, Qe7 prevents the bishop on f8 from moving, which in turn stops Black from castling at a time when he'd prefer to get his king away from the center.
8. Nc3
Yes, Morphy could have won material by playing Qxb7 or Bxf7 (in the latter case, Black cannot play Qxf7, as White's reply of Qxb7 now would win the a8 rook), and this material would have been sufficient to win the game.
However, Morphy preferred to concentrate on development, coordinating his pieces for a direct assault on Black's king. White's better piece coordination and faster development forces Black to stay on the defensive.
8. ... c6
Black uses his pawn to prevent White's knight from jumping into the b5 or d5 squares. This is the best way to defend against an advancing knight -- use pawns to take away its potential forward squares.
9. Bg5
Pinning the knight, which continues to keep Black's pieces undeveloped and badly coordinated.
9. ... b5
Black attempts to dislodge the White bishop, and brings us to our next diagram.
