Chess Opening Theory/1. e4/1...e6/2. d4/2...d5/3. Nd2/3...Be7

This is in principle a waiting move. Black is waiting for White to reveal their set-up without committing to any irreversible moves, since the bishop can be repositioned quite flexibly later in the game.

4. Ngf3 is the main line. It is not the most flexible move, since White often wants to position their knight on e2 instead and also reserve the possibility of playing f2-f4. This line resembles the Universal system of the closed Tarrasch. The big difference being that White didn't have the opportunity to play 7. Ne2 and Black has already committed their bishop to e7.

4. Bd3 is White's main alternative. Contrary to the general advice of developing "knights before bishops," this move is considered to be more flexible than 4. Ngf3, since the bishop is well-placed on d3 more often than the knight is on f3. This move also turns 4...Nf6? into a mistake after 5. e5 Nfd7 6. Qg4! ± and White has plenty of attacking opportunities due to Black's pieces all being stuck on the queenside. Instead, Black should opt for 4...c5 and the game continues normally.

4. e5 is a sideline. White commits their pawn to e5, locking down the centre and steering the game into a direction similar to the closed Tarrasch, but with Black's knight somewhat stuck on g8. After 4...c5 White has an important fork in the road: they can play 5. c3 for a quieter game, or they can play 5. Qg4 for a more dangerous attacking game.

The Morozevich variation received a lot of attention in the 1970s and onwards, becoming a solid option for Black against the Tarrasch.^[1] This variation gets its name from grandmaster Alexander Morozevich, who introduced his aggressive playing style into the opening.^[2]

1. e4 e6 2. d4 d5 3. Nd2 Be7

Bad Bishop Chess Channel. "French Defense: Morozevich Variation (for Black)". www.youtube.com.

Wikipedia has related information at French Defense.