French defense: Key ideas on the Winawer I am beginning to study the Winawer variation (1.e4 e6 2.d4 d5 3.Nc3 Bb4), mainly because the only line I actually know against 3.Nc3 is the Rubinstein (3...dxe4), which is very dry and boring to my taste. 

I have studied the main line, and I'm beginning to appreciate the key ideas of the Winawer, but still I would like to ask to more experienced French defense players what are the key ideas, notes and/or concepts I should keep in mind while studying (and playing!) this sharp variation? This will make me study with better fundamentals and play with more precision, I hope.
Since the question is posed very generally, I'll only give a rough overview of some of the more typical ideas behind the Winawer for black. I'll mostly focus on positions after 4.e5 as that is the key line allowing white to fight for an advantage out of the opening. 

It must be noted that the ideas tend to vary somewhat greatly depending on which specific variation of the Winawer we consider, as there are quite different ways the opening can continue after 1.e4 e6 2.d4 d5 3.Nc3 Bb4 4.e5, namely:


The advance variation with 4...c5,
The other advance variation with 4...Ne7,
The Petrosian variation with 4...Qd7, etc.




Let's start with 4...c5, and consider the following typical position of the Winawer, arising after 5.a3 Bxc3+ 6.bxc3:

 

Strategic assessment:


First of, black has willingly decided to give up their dark-squared bishop, so white has the bishop pair. 
In turn, this has allowed black to damage white's pawn structure.
In particular, the pawn structure favours black in the endgame thanks to white's isolated a-pawn and the doubled c-pawns.
Although black is clearly behind in space control, the central pawn structure makes it very difficult for white to actually open up the position. The position remaining closed favours black as white has the bishop pair and is getting their pieces more effectively developed than black is able to. 
Black's main two weaknesses are: the c8 bishop having no simple prospect of getting effectively developed, and the highly weakened kingside caused by giving up the f8 bishop and the inability to establish the kingside knight on the ideal f6 square, which is prevented by white's early e5.
The two aforementioned aspects are what makes the advance variation with 4.e5 favourable for white, as these weaknesses offer white a tangible and concrete edge to play with. 
From black's perspective, these difficulties in parrying a kingside attack or finishing development effectively means, on the one hand, black has to be ready to opt for dynamic remedies to solve their problems, such as the poisoned g7-pawn continuation to answer white's early Qg4. On the other hand, black has to know and recognise a lot of positional combinations in order to defend against white's various attacking options. These combinations are typically in the form of very subtle manoeuvres that at first glance appear to completely undermine all opening principles. 


With that, let's have a look at some of these subtleties you need to be definitely aware of when playing the Winawer. To understand their necessity, we must first develop a feel for where white's best attacking chances lie. Consider again the following typical structure, but this time after black's best move 6th move, 6...Ne7:

White has three main active continuations here:


7.a4: with the idea to develop the c1 bishop to a3 and quickly exert pressure on black's dark-square weaknesses. 
7.h4: a quick advance on the kingside to either play h5 (often followed by g4) to boot black's e7  knight from the key light-square posts g6-f5. Additionally, advancing the pawn to h6 is also possible, to either damage black's pawn structure after gxh6 or to further weaken the dark-squares after g6.
7.Qg4: immediately exploiting black's weakened kingside and in particular the undefended g7 pawn.




Important typical manoeuvres for black:

To prevent an early 7.a4, black has the option to play 6...Qa5 (instead of 6...Ne7) with tempo as c3 is attacked, with the idea to block next the a-pawn with Qa4! Indeed this looks dubious at first as the queen is being used as a blockader, but in fact on top of the blockade, the black queen is also well-established for creating counter-attacks on the queenside and in the center. 



After 6...Ne7, in reply to white's 7.Qg4, as opposed to the more risky lines with 7...O-O, black has the option of playing the Poisoned Pawn variation, which after 7...Nbc6 entails: Sacrificing the g7 pawn for a quicker development, queenside castling, and dynamic play:



A typical continuation is: 

 [title "Poisoned Pawn Variation:"]
 [fen ""]
 [startply "13"]

 1. e4 e6 2. d4 d5 3. Nc3 Bb4 4. e5 c5 5. a3 Bxc3+ 6. bxc3 Ne7 7. Qg4 Nbc6 8. Qxg7 Rg8 9. Qxh7 cxd4 10. Ne2 Qc7 11. f4 dxc3 12. Qd3 d4 13. Nxd4 Nxd4 14. Qxd4 Bd7 15. Rg1 Nf5 16. Qf2 {black is a pawn down but has decent positional compensation thanks to white's compromised king position, the well-established knight on f5 and the white's g2 pawn against which black can continue to mount pressure with Qc6-Qd5-Bc6.}


The Petrosian variation starting from 4...Qd7 (instead of 4...c5) looks really dubious at first since: we're developing our queen by blocking our own bishop, and even before having developped our minor pieces. But in fact, Qd7 is a really useful multi-purpose move: it prevents Bb5+ and prepared to trade the light-square bishops with b6-Ba6 and it indirectly defends the g7 pawn against Qg4. Since with the queen on d7, Qg4 can be replied with f5, a discovered defense if you will :)



And the list goes on... But you see the pattern behind these subtleties: they are all backed by highly concrete ideas and thus, far from a mere application of opening principles. These are things you need to take into account if you really want to adopt playing the Winawer, these systems are not everyone's cup of tea.



Summarised notes

Again, this was simply a rough overview, attempting to bring to your attention the kinds of positions and manoeuvres you need to get yourself accustomed to if you want to start playing the Winawer. Let's finish by summarising mostly black's trumps in the Winawer:


Black ensures the center remains closed with early trade on c3 leading to the typical pawn structure of c2-c3-d4-e5 for white and (c5)-d5-e6 for black. White has the lead in space and has the bishop pair, so it's vital to keep the position closed, unless a liquidation is possible.
The early trade on c3 gives black, statically, the better pawn structure and it leaves white with an isolated a-pawn and doubled c-pawns. Thus, typically, early liquidations towards an endgame are better for black, but of course white should not allow that.
With all of white's central pawns fixed on dark-squares, black can undermine the pawn chain with c5 and can easily mount further pressure on c3 and d4 pawns (moves such as Nbc6, Ne7-Nf5, Qb6, Qa5).
However, dark-squares can also easily be black's undoing in the Winawer, specially, if black is unable to create a clear counter-play and white manages to force further dark-square weaknesses on the kingside, then white's queen and dark-squared bishop coordination can become dominant and decisive. See e.g. Hou Yifan's instructive win against Kramnik in the Winawer, an absolute dark-square domination.
As for instance advertised in the Poisoned pawn variation, black commonly has the option of also castling queenside to finish development, and play actively.
Black's main worries in the Winawer are: how to parry the induced dark-square weaknesses, and how to complete development, in particular, the king position and the locked out c8 bishop. If white plays passively early on, and allows you to comfortably resolve these, then you'll have equalised, if not even a bit better because of the pawn structure.
Exploit white's exposed queenside in order to create counter-play by targeting white's pawns, in particular the a4 square, which if occupied by either the c8 (Bd7-Ba4) bishop or queen (Qa5-Qa4), c2 can be targeted and more importantly black prevents the a4-Ba3 setup for white as discussed earlier.
Also note that lines where white sacrifices their e4 pawn do not really work, and you need to be ready to refute them. For instance, after 1. e4 e6 2. d4 d5 3. Nc3 Bb4 4. a3, (instead of e5), black stands even slightly better after: 4...Bxc3+ 5. bxc3 dxe4 6. Qg4 Nf6 7. Qxg7 Rg8. Similarly for 4.Ne2, which allows black to comfortably equalise: 4. Ne2 dxe4 5. a3 Be7 6. Nxe4 Nf6.
As a whole, the Winawer can be a very dynamic weapon to play for a win as it can lead to highly imbalanced positions and still bears some hanging fruits to be discovered.
As a general recommendation for learning about ideas underlying a specific opening, make sure to spend a lot of time studying games of experts of the opening you're interested in. Look into the games of for example, D. Bronstein, Y. Seirawan, T.V. Petrosian, V. Smyslov, F. Caruana and N. Zhukova.