The Slav Defense does a lot of things for Black that the Queen's Gambit Declined does. It has the same one major advantage that the Caro-Kann has over the French Defense. Easy development for the light square Bishop.
As Black, this opening gives you free and easy development and a good solid position. You will have scope for imaginative play in many lines.
As White, your most ambitious way to meet this opening is to simply develop your pieces. 3.Nf3 is by far White's most common reply. Then there's the Exchange Variation and 3.Nc3.
Maybe you want to release the tension straight away and slowly build. You can consider 3.cxd5 cxd5 and you're in the Exchange Variation. Now you just develop in a routine manner and you will get a playable position.
It may be harder to get an advantage but then again you're safe and solid. It's still possible for either side to maneuver their way to an advantage.
As White you may try to plant a Knight on c5 to pressurize Black's Queenside. Maybe you can force an error and create an enduring weakness. If you're playing with Black and you feel adventurous you can actually attack on the Kingside in some lines. You can generate a surprising amount of pressure.
The Argentinean Defense starts with 3...dxc4. As always with this move Black is playing for a 4 vs 2 majority on the a, b and c-files. He hopes that this pressure will give him the upper hand in the struggle.
White can respond in 3 ways. He can play 4.a4 to deny Black's c4-pawn support. He will then try to round up the pawn before Black can establish a lifeline to it. White also has 4.e4 to give him a big center. Now ...b5 is allowed and Black can consolidate his pawn. The e4-pawn hinders tactics against the a8-Rook.
4.e3 is the critical test of 3...dxc4. Black's best bet is to let the c4-pawn go. He can try to hold it but the game will take on a sharp tactical nature. It is easy to lose the thread of the game without preparation.
You can choose a dynamic game, rich in attacking ideas with the Schallop Variation. You play 4...Bf5 with the intention to shut the door behind the Bishop with ...e6. White can force the exchange with Nh5 if he wants but in that case the Bishop can drop back to g6 with the thinly veiled threat of opening the h-file against the White King.
You're okay with the exchange. All your central pawns are on light squares anyway so your dark square Bishop is the one you really need. This Bishop often forms a battery with the Queen on d6 and c7 looking at that h2-square.
With the h-Rook possibly operating out of a half-open h-file the enemy King is feeling uneasy. Otherwise if the light square Bishop has not been exchanged then he has a great game on the b1-h7 diagonal.
White has chances too in the Schallop. The White Queen can go to b3 and threaten b7 with the light square Bishop absent from c8. Black has to play carefully to prevent you from undermining his Queenside.
We reach the Chebanenko Variation (Chameleon Variation) after 4...a6. You play this move with the intention of strengthening ...b5 which will come after ...dxc4. White is forced to either take on d5 or play 5.c5 to prevent this from happening.
After the main move, 5.c5, your two options are 5...Bf5 or 5...Nbd7. The latter move locks in the Bishop on c8 in direct violation of Slav principles. 5...Bf5 is the preferred move for this reason.
As White you would play 6.Bf4 at this point to reinforce control of e5 and d6. Black can now play ...Nbd7 without hesitation and with a possible ...e5 break in mind. Now you can choose between 7. h3 or 7.e3.
7.h3 prevents Black from forcing the exchange of White's important dark square Bishop. It is slower than 7.e3 and allows Black a more comfortable game.
The Schlechter Variation is a way for Black to get a playable, if slightly passive game. 5.e3 is almost obligatory and after 5...Bg7 6.Be2 we enter the Main Line.
White seems to have the upper hand in all lines winning his fair share of games. Inventive play to open Black up is needed though. The resolution of the tension between c4 and d5 has a major bearing on the character of the game.
If White plays cxd5 at some point then after the recapture ...cxd5 Black closes up the Kingside with ...e6 and the game resembles a QGD. If Black takes on c4 then the game opens up as the center pawns are cleared with White often taking over the d-file.
The Alekhine Variation sees White play 5.e3. In doing so you will allow Black to play ...b5. And yet the pawn can be won unless Black wants to get into some difficult lines. After 6.a4 the pawn is once again under threat as the c3-Knight also attacks the b5-square.
Black can hold the c4-pawn by challenging this Knight with 6...Nd5. Things get a little wild and it's not clear if Black is better after the complications even if he does hold the pawn.
As Black you will usually just play 6...b4, kicking the Knight back to a2. Your c4-pawn will fall sure but you can retain a hold on the Queenside with ...a5 in the next couple of moves. You can then play ...e6 to dent the ambitions of the c4-Bishop, getting a good game.
The Alapin Variation is the rather radical looking response to 4...dxc4. The move cuts off the c4-pawn's lifeline but it leaves b4 weak and ripe for invasion.
This is a price White is willing to pay for a good center. Black's Queenside Knight often shows up on b4 sooner or later. Or maybe the dark square Bishop in some lines of the Czech Defense.
The Main Line shows White winning the battle for space in the center. You will also have play on the Kingside. As Black you get a solid, compact position with counter play through your prize on b4. You also have the thematic Slav/Semi-Slav breaks, ...c5 and ...e5, in reserve.
If you're not happy with White's positional assets in the Main Line, you can liven things up after 5...Bf5/5...e6 with 6.Ne5 instead of the more methodical 6.e3. Now things may get tactical as both sides see their centers liquidated for an open battle.
The Steiner Variation is a dynamic response to the Alapin as Black comes up with 5...Bg4. White can answer this simply with 6.e3 or 6.e4. He accepts the pin for the time being and picks up the c4-pawn before anything crazy happens. Simple development will follow.
If you're feeling adventurous with the White pieces you can play 6.Ne5 where a sharper game could well follow. After a tactical exchange c4 is picked off, the Queens often come off and the White King can take up a safe post in the center.
In many lines White finds himself with a 4-3 majority on the Kingside vs a 3-2 majority on the Queenside for Black. Symmetry also appears in some of the tactical lines.
You can play the Smyslov Variation (5...Na6) as Black if you don't want anything silly happening as you develop. You will get a solid structure and easy development in all lines. Your pieces won't be at the gates of White's citadel by any means but you'll be able to grow into the game.
You will see your pawn shell take a kind of Semi-Slav-esque formation with your c and e-pawns on your third rank and the rest on their home squares. You can safely castle and prepare for a positional struggle. You're backing yourself to outplay your opponent over 40-50 moves.
With White you can meet the Smyslov in three ways. You can go for quick development with 6.e3 or 6.e4. The former for solidity, the latter for more space. Or you can take Black on in a tactical duel with 6.Ne5.
This third move comes with a health warning though. White does not score well with it. Black can force the trade of Queens while liquidating the center. It's difficult for White to catch up in development with his King blocking up the center.
The Soultanbeieff Variation is another approach to the Alapin. This time as Black you will waste no time closing the a2-g8 diagonal to any potential c4-Bishop.
The main idea here is to play ...c5 at the right moment, pressurizing d4. You can leave White with an isolated d-pawn. Your Knights will be well posted on d5 and c6.
As White you can choose to accept the isolated pawn. You will trust in your ability to use your space advantage and active pieces to worry Black. If you don't like the sound of that you can always pin the pawn to his Queen with Rd1, giving yourself time to recapture with the Knight.
The Czech Defense is the Main Line of the Slav. You play 5...Bf5 to prevent White from playing e4. White wants this move as it gives him his big center. It also allows him free and easy development.
If you can restrict White to e3 it makes things a little more difficult. He has to delay his Kingside development in order to avoid getting a passive dark square Bishop. The longer you keep the King in the center, the more chances you have to harass him via the open Queenside.
The Czech Defense has a number of variations and sidelines. Three stand out from the rest. They are the Dutch Variation, Krause Attack and Bled Attack.
The Dutch Variation is the Main Line of the Czech Defense. That makes it the most important line in the Slav Defense. You play 6.e3 against the Czech but this is not as modest as it seems.
The main continuation for Black is 6...e6. You're going to finish up with that e3-pawn on e5. The Knight will be expelled from f6. Your pieces will be directed against Black's Kingside. As Black you will have to neutralize White's attack if you can and expand on the Queenside.
You also have options other than 6...e6. You can go with 6...Na6 with your Bishop to drop in at b4. These lines can get exciting with the Queen coming to a5 and White responding with possible pawn storms on the Kingside. Other tries are 6...Bd3 or 6...Nbd7.
The Krause Attack is another try for White against the Czech Defense. With 6.Ne5 you can take the c4-pawn without committing your Bishop to the Queenside. This allows the option of fianchettoing and supporting e4 from g2.
You could also simply continue with f3 and e4 getting that big center and shutting the enemy light square Bishop out of the game for a long time. In this line the Bishop can take on c4 and get a good look at f7.
The two main replies to the Krause are 6...Nbd7 and 6...e6. You could also try 6...Na6 if you face this position with the Black pieces. 6....Nbd7 has seen many games where White fianchettoes. Black often castles long and gambles that he can open up the enemy King first.
6...e6 has two main branches. You either try the ...c5 break and trade Queens or go with ...Bb4 followed by ...Nd5. Here you sacrifice a piece for three pawns, the safer King and a much more solid structure.
White's third most common answer to the Czech Defense is the Bled Attack. 6.Nh4 gets this one going and Black has to decide whether he wants to preserve his light square Bishop.
White usually allows the exchange with 6...e6. After the recapture he will have a pawn on f5 doing the job his Bishop had been doing. White will end up having to settle for e3 in these lines as the f5-pawn is usually reinforced by ...g6.
As Black your other main alternatives on move 6 involve retreating the Bishop. The most popular of these is believe it or not 6...Bc8. It's not really a loss of tempo as the Knight is out of position on h5. 6...Bg6 doesn't do much in the fight for e4 as after the exchange, Black does not have a pawn controlling e4.
The Slav Defense is a really solid way for Black to play while retaining realistic winning chances. It has something to offer for all kinds of players. Positional grinding or tactical shoot-outs, whatever's your thing.