Chess Opening Theory/1. d4/1...Nf6/2. c4/2...e5/3. dxe5/3...Ne4/4. Nf3

The move 4.Nf3 develops a piece and covers the sensitive d2-square. Tseitlin considers 4...Bb4+ to be the best reply, but the most common move is still 4...Nc6 and both moves often transpose anyway.

4...Nc6
1.d4 Nf6 2.c4 e5 3.dxe5 Ne4 4.Nf3 Nc6

After the continuation 5.Nbd2 Bb4 6.a3 Nxd2 7.Bxd2 Bxd2+ 8.Qxd2 Qe7 9.Qc3 much will depend on whether Black can get their pawn back or not, and in what conditions. Tseitlin gives a line starting by 9...O-O as leading to a White advantage, but it is not clear that Black's king belongs to the kingside, as Lalic explains: "The plan of castling queenside enables Black to play for g7–g5, intending to undermine the defence of the e-pawn or simply to launch an attack if White has still had the courage to castle kingside."

Black has also tried 5...Nc5, in order to let the Nd2 on its bad square where it blocks the Bc1. In a top-level game Alekhine chose to dominate the light squares with the development Bf1–g2, and after 6.g3 Qe7 7.Bg2 g6 they went for the paradoxical 8.Nb1!, as Tseitlin comments: "This move, though astonishing at first sight, is in fact perfectly logical. Once Black has clearly revealed their intention to develop their bishop on g7, White no longer needs to reckon with any action on the e1–a5 diagonal. Therefore they has no reason to refrain from bringing their knight to the dominating d5-square." The position of the knight in d5 latter forced Black to play c6–c5, creating a weak d6-pawn that ultimately caused Black's defeat. Black can improve with 6.g3 d6 7.exd6 Bxd6 8.Bg2 O-O 9.O-O Qf6 10.Nb3 Be6 11.Nxc5 Bxc5 12.Qa4 Bg4!, retaining good attacking possibilities for the gambit pawn. Similarly after 10.Rb1 (instead of 10.Nb3) Lalic notes that: "Black has some compensation for the sacrificed pawn, since the white knight on d2 is awkwardly placed and Black has a definite lead in development, although this will be useless if White is allowed to consolidate their position, as they has no weaknesses." Another idea for Black is to castle long, with 6.g3 d6 7.exd6 Qxd6 8.Bg2 Bf5 9.O-O O-O-O 10.a3 and now the move 10...Qf6 is essential to hinder the dangerous advance b2–b4.

Instead of 6.g3, White has also replied to 5...Nc5 with the funny 6.Nb3 and after the reply 6...Ne4 White has gained the moves Nb1–d2–b3 for free, but whether it is an advantage is unclear. White can make use of their better control of the c5-square with 7.a3 d6 8.Qd5 Bf5 9.exd6 Nxd6 10.Nc5 threatening 11.Nxb7. White can also opt for the flexible 6.a3 (threatening 7.b4) 6...Qe7 7.e3 Nxe5 8.Qc2 a5 9.b3 b6 with roughly equal chances.

Better than 5.Nbd2 is the preparatory move 5.a3!, hindering any Bf8–b4+ or Nc6–b4 idea. Now Black has to do something about the Qd1–c2 threat. They can try 5...d6 hoping to attract their opponent into something like 6.exd6 Bxd6 7.Nbd2 Bf5 8.e3 Qf6 9.Nxe4 Bxe4 10.Bd3 where Black retains some play. But White should not oblige and must play the strongest 6.Qc2 so that after the sharp 6...Bf5 7.Nc3 Nxf2 8.Qxf5 Nxh1 Black will not be able to save their Nh1 and White has the advantage. Even if Black reacts with the more prudent 6...Nc5 7.b4 Ne6 8.Bb2 dxe5 9.e3 f6 10.Bd3 g6 11.Nc3 Bg7 White has the initiative and Black can only hope to gradually neutralise White's strong pressure.

4...Bb4+
1.d4 Nf6 2.c4 e5 3.dxe5 Ne4 4.Nf3 Bb4+

Instead of 4...Nc6, Black does best to include 4...Bb4+ which Tseitlin considers "the simplest and most reliable course", as White then has to choose between different difficulties. After 5.Bd2 Nxd2 6.Nbxd2 Nc6 7.a3 Bxd2+ 8.Qxd2 Qe7 9.Qc3 it seems White has everything covered, but Borik thinks Black will eventually be able to regain their pawn with the plan b6/Bb7/O-O-O/Rde8/g5/g4, with an initiative on the kingside. Tseitlin does not agree and prefers the continuation 7...Bf8! (instead of 7...Bxd2+) 8.Qc2 g6 9.Qc3 Bg7 so that "Black not only makes sure of recovering the gambit pawn, they also preserve the advantage of the bishop pair". Black's chances are to be preferred.

So it is better for White to keep the bishop with 5.Nbd2 Nc6 6.a3 and now Black can easily get confused by the move-order. After the natural 6...Nxd2 7.Bxd2 Bxd2+ 8.Qxd2 Qe7 9.Qc3 the game has transposed in the same position as after 5.Bd2, but White can also try 6...Nxd2 7.axb4! Nxf3+ 8.gxf3 Nxe5 9.Rg1 Qe7 10.Ra3! with a strong initiative. White can even retain their bishop with 6...Nxd2 7.Nxd2 and now Borik recommends 7...Bf8 with difficult play for Black as they are not certain to gain their pawn back. To avoid these possibilities Lalic advises the move-order 6...Bxd2+ 7.Bxd2 Nxd2 8.Qxd2 Qe7, but does not mention the possibility of White answering 6...Bxd2+ with 7.Nxd2. A possible improvement for Black would be 5...d5 when Tseitlin sees sufficient compensation for the pawn in all lines.

Theory table
1.d4 Nf6 2.c4 e5 3.dxe5 Ne4 4.Nf3