Chess Opening Theory/1. e4/1...d5/2. exd5/2...Qxd5/3. Nc3/3...Qa5

= Scandinavian Defence =

By moving the queen to a5, she cannot be attacked again, she can pin the knight if White dares to advance their d pawn, and she can later move to b6 to target the b2 pawn. This also sets the stage for developing black's pieces while setting up some threats, a much better option than ...Qd8. White's main plans here are Nf3 and d4, continuing to control the center and developing his pieces. Black can give their queen escape squares with a timely ...c6 in case it gets into danger, and can develop their pieces and go for an e7-e5 advance.

Other than 4. Nf3 and 4. d4, white also has the option of 4. Bc4, another simple development move. Most of these lines generally transpose into each other and the Scandinavian in itself doesn't have all that much theory due to it not being played too often. White generally maintains a small advantage in Scandinavian Defense middlegames, but black can generally hold the balance if they play correctly (white has a ~57% score, which is not particularly good for black, but it's not particularly bad either).

Even 4. g3!? and 4. Be2 (a rather safe option) are completely playable and score similarly to the main moves.

White also has the option of 4. b4!? (the Leonhardt Gambit), a gambit that gives white an opportunity to attack with moves like Rb1, a4, and Ba3, while maintaining a lead in development. While theory suggests that white doesn't get enough compensation for the pawn, computer analysis with more recent engines on high depths have shown that white still gets good compensation. Furthermore, over the board, it is quite hard to prove that black can maintain the extra pawn safely into a better endgame due to the attacking opportunities that white gets. Historically, Lasker, Capablanca, and Keres have attempted this line with decent success:

Lasker vs Fortujin 1-0

Capablanca vs Portela 1/2-1/2

Keres vs Kibbermann 1-0

Other than these lines, occasional moves like Rb1?!, a3!? (intending b4 next), h3?!, Nge2, Qe2?!, and f4?! have been tried, but there is little data on those moves, so a definitive conclusion cannot be reached about that. Most of these moves do seem to be playable according to engine analysis, however (Qe2 and f4, not so much).

Note that 4. Bb5+? just loses a tempo to 4... c6!. Don't just check your opponent for no reason (a common beginner mistake). Attempting scholar's mate is also unlikely to succeed, because black can just play e6.