OK, so I'm aware no one asked for this in Opening suggestions forums but I practically had it written so...
Introduction To The French Defense
The french defense begins after 1. e4 e6. The purpose of this first move to to get a firm pawn on d4. Often black can't mantain a central pawn in symetrical kings pawn openings, as white will always have the intative. Immediately, an unbalance in the position is created by the French.
Lets examine some of those variations.
Advanced Variation
Black should remember not to fall into the trap of counting peices and deciding to take twice on d4 (once is OK, often the best.) from this position.
There's a few choices here.
...Bd7
...Nh6
taking once ...cxd4, undermining the white centre.
are the two most common moves.
Bd7 and Nh6 both exert pressure on the centre indirectly. If you solved the puzzle above, you can see that with a bishop on d7, that tactic won't work, so black is now threatening to win a central pawn.
Nh6 has the idea of coming to f5 or possibly g4 to pressure the centre.
Lets say we play Nh6. You might be wondering if white shouldn't just take on h6 with their c1 bishop, giving black isolated doubled pawns when s/he recaptures, and eliminating another attacker on the centre.
Logical, but wrong.
Strategies And Guidelines
With a closed centre, both players should aim for play on the wings.
f6 is a move black often plays to undermine the centre.
Black can often have an attack on the queenside. His/Her rook often comes to c8 and the queenside pawns advance, often hoping to attack c3 with a pawn on b4.
To stop this, white often plays a3 and b4. In fact, a3 is a more popular move than Bd3, but I wanted to show the discovered attack tactic. a3 and b4 can slow down black's queenside play.
White should aim for attacks on the kingside, normally trying to attack e6 with a pawn on f5. Even though white normally castles kingside, s/he can normally get away with moving g4, particularly if it attacks a knight on f5.
To try and stop kingside play, black can try and move kingside pawns a few forward. There are a few choices: f5, g6, or g5.
f5 has the disadvantage of no longer being a break on f6 to continue to undermine the white centre.
g6 is sometimes a good move, slowing down queenside attacks, but leaving holes on f6 and h6.
g5 leaves the same holes but if defended by h6 can stop a lot of kingside play.