Disclaimer: I will ramble some about some things.
I don't know what level you are really at, how much knowledge you have already etc, so please don't be offended if you already know most of this and it sounds like I'm selling your abilities short or w/e - I'm just trying to be thorough.
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To give the basic reasoning as to why tactics training is so important, I'll quote Réti: "A knowledge of tactics is the foundation of positional play".
Everything you know as guidelines and positional ideas takes a backseat to tactics (pawns are strongest in a phalanx; but if hg3: captures a queen, you're going to do just that);
Strong positional play (say, creating a weak square in the opponents camp and parking a minor piece on it) is generally not sufficient for a win unless you find a tactical shot - enabled by your positional superiority;
In the first place, you *just won't* understand what good squares for a piece are (or why), if you are unaware of typical tactical motifs.
The most basic example off the top of my head is the Légal Trap:
[ 1.e4 e5 2.Nf3 Nc6 3.Bc4 d6 4.Nc3 Bg5?! 5.h3 ]
.. So far this looks like a bunch of normal moves.
Now black is going to remember that:
- moving the bishop back on the same diagonal is a waste of tempo (just gave white h3 for free),
- ..Bf3: concedes the bishop pair which is suboptimal,
so 5...Bh5 is the most logical move.
.. But! Here, positional guidelines have to take a backseat to tactical considerations.
After 5. ..Bh5, white has the shot 6.Ne5:!
Since 6. ..Bd1: 7.Bf7:+ Ke7 8.Nd5# ends the game, black will emerge from the complications a pawn down.
I think this is key - I simply don't have that kind of general knowledge at the moment.
Just look as far as you can!
If you don't understand the concept of a weak colour complex, look for weak squares, or even just hanging pieces.
If you don't see any immediate win, look for "possible future things" such as a rook standing in a long bishop diagonal with a pawn in between, queen and king at forking distance, etc
If you have no real idea of what kind of plan a person should have, at least try to rule out some moves which you'd never play (eg "this pawn move would be bad due to locking in my bishop" or w/e)
With some time of playing the Sicilian, analysing Sicilian games, and solving tactics puzzles which arose from the Sicilian, you are going to notice typical sacrifices on c3, d5, e6.
Even if you're at no point actually aware of a given position being a Sicilian one, you will have seen similar structures and understand what to look for.
If you have no general knowledge of openings and the arising pawn structures, then you can also just forget about this thought - and consider other things.
Eg: "White has this strong attack on the board, and is supposed to be winning in this position (has a tactical shot hidden somewhere), so Black blundered somewhere before. But what was Blacks general idea in giving White this attack? Does he have some compensation, or must he have played 15 terrible moves in succession?"
In the Puzzles taken from master games rather than dumb studies, you will generally find something like "White has a surprising rook sacrifice leading to a mate in 4, but *if he didn't have that*, Black would be able to break through on the Queenside via this-that-this; the defensive positioning around the Black King with that Knight on g7 is exceptionally strong and would be unbreakable without this rook sacrifice; I should make note and try to apply similar in my games".
Thoughts like this arise "naturally" when actually considering the entire board, rather than just looking for a winning combination.
I know that in the Najdorf, a d5 break is supposed to be one of Black's key plans. But because I've never played the Najdorf, you could show me a Najdorf-like position and I wouldn't recognize it, so the information is essentially useless.
The base idea behind learning these guidelines is that you get the raw answer to a question - and then try to find out why this is a guideline, how far it goes, when it actually applies, etc.
What is 1. ..c5 good for?
Compared to 1. ..e5, it doesn't really help you develop your pieces (Bf8 still locked in), and it controls less relevant squares (d4+b4 instead of d4+f4). So why would anyone play this move?
Because white, to abuse the lead in development you just served him on a platter, has to open lines; else his initiative is going to fizzle out sooner or later.
After the natural moves 2.Nf3 Nc6 3.d4 cd4: 4.Nd4:, white has achieved his goal - he broke open the position, established a dominating knight on a central outpost, got a spatial advantage, and is ready to develop his minor pieces (look at those bishops compared to black's!).
However, doing so came at a price which would not happen after x.d4 in a e4 v e5 pawnstructure: White gave up one of his central pawns for black's c-pawn. If black succeeds in freeing his position, he will enjoy a central pawn majority, which would promise him a good game.
This is the very idea of the Sicilian. Black accepts that he will be behind in development (and in a slightly cramped structure) for the foreseeable future;
Hoping to be able to at some point play ..d5, since after ed5: xd5: (followed by ..e5) Black would gain strong central presence, which white will forever be unable to challenge - at that point it wouldn't just be a fight for equality, but secure him superior prospects.
If you understand the ideas of central pawn majority, pawn breaks, how to play in a cramped position, etc - then you will get the idea of playing ..d5 at some point yourself; it doesn't really matter whether you recognize that the given position is a Najdorf structure.
If you understand this, then you will also know that ..d5 is the main underlying idea of *every* Sicilian, rather than being unique to the Najdorf - it's just easier to achieve in some Sicilians than in others.
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.. You know that a Rook is generally "worth more" than a Bishop, but why is that so? Maybe you heard the saying "Flank attacks get countered by central breakthroughs", or know that the bishop pair is strong, or are aware that control over the center is important, .. but why, why, why?
These guidelines give you some immediate successes if you just blindly follow them, but it is important that at some point you sit down and try to figure out WHY EXACTLY they give you those successes; so you can understand when the guidelines don't apply, or how to abuse them in positions where they aren't obvious -
These are the baselines of positional play; understanding that your spatial advantage can only persist if the opponent doesn't get to play his f7-f6 pawnbreak, noticing that you cannot stop him from doing that for long, and thus swinging a rook on the completely closed-off e-file, as you know black will sooner or later push ..f6, opening up the center, and THEN that rook is going to be strong.
EDIT: So, does this indicate that I should be studying pawn structures? Or perhaps light study of all (!) openings? Or something else?
Studying pawn structures and their implications is never a bad idea! Same with rough reasoning for various different opening approaches. At 1500 level, probably neither is necessary, though. As said, most of these games are raw tactics :P
Before you misunderstand: These aren't "You have to do this" things, but just general ideas how to make 'tactics training' via puzzles (which, at its core, is just "Get a position and find the winning move", a lot) less bland and more relevant for real play - putting you in the mindset of "If I had gotten this position during OTB play, what would be my thoughts?".
The point is that many (most?) people, when doing tactics puzzles, sit down and just go "Okay I have to win this position. How do I do that? Sac there? Sac here? Sac this? Sac that?"; while if they were to get 1:1 the same position at the board, the sac wouldn't even enter their mind at all and they'd just play some developing move or similar.
So instead, you try to forget that there's a win somewhere, and just look: What are my opponent's plans, what are my plans, what should I prevent, where do my pieces want to go - and often, you'll see "Oh there is a weak square! It's currently protected by the opponent's knight, but if I could plant my knight there, it would have a stellar outpost and threaten mate on f7.. wait, I can capture the defending knight with my rook, would that be worth it? Yes, he can only take the Knight back with his Queen, and then the mate/queen fork is a killing blow" - this would be a thought process you could have OTB.
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Again, I am going to have a hard time telling you what you need to work on without playing a bunch of matches (all of CC, long, and short time limits), seeing you analyze differing positions, and noticing how you handle certain pawn structures.
If you know the basic moves of your repertoire of choice, understand the underlying ideas of the openings you play, are aware of the basic opening rules, have an idea about the middlegame plans both in your openings as well as in "topical" pawn structures (IQP, pawnchains, semi open files, ..), know what a weak colour complex and what weak pieces are, can perform basic attacks (both piece attacks / pawnstorms) and have the groundworks of endgames down (when K+P v K is won / when it's drawn; how to play Rook endgames) - plus are able to solve basic tactics in your games (don't blunder pieces/pawns away, see check-sac-fork), then..
I don't know, buy the Khmelnitsky book :P