What is the best way to improve understanding of strategy and endgames?

Yeah it was a tossup between f4 and Bf4 for me. I think both moves are great here.

As usual, its probably a preference of "style"

Thank you for all the tips Deranged and IMBacon. I just played 2 games, the first one was decided by tactics before strategy even got into the picture properly. The second game was EXACTLY the type of game I struggle with. 

I find it hard to make use of the centre if my opponent decides to go for a hypermodern setup. I simply dont know which pawns to push when and where my pieces belong, when there are so many options 

Quick analysis: I realize I played right into my opponents` hands. I shouldn`t have captured en passant  on move 11, why give up a pawn i spent several tempos getting to e4.. if it had stayed there the g1 knight would have faced development problems, any by extension the h1 rook as well.. I should instead have gone for a plan involving Bf5, and the pawn pushes b5, a5 and d4

9...d4 was more appropriate - seems like black has a very big advantage. 

The problem is that playing moves like ...Qd6 and ...a6 don't really help you that much. 9...a5 was also a very useful move. 

Note that taking e.p on f3 freed white's bishop and reduces your space advantage - if you have a tactic, that's all good, but you didn't play any in the game.

Overall, you can only learn strategy by thoroughly looking through master games, and getting a sense of how they play the positions. 

The issue here is that without strategy, you can't make tactics, and without strategy, you get caught in tactics more easily.

Those that can, do.  Those that cant, teach.  Those that cant teach, consult.  Those that cant consult, copy and paste.

Let’s take a look at when it makes sense to castle opposite sides and when it doesn't.

You should castle on the opposite side when at least one of the following factors is true:

1. When you are up in development and your opponent has already castled, you should consider castling in the opposite side. That way you will have a clear game plan and will also be able to capitalize on your development advantage.
2. When you have a damaged pawn structure (doubled paws, missing pawns, far advanced pawns, etc.) on one of the sides you should consider castling on the other side.
3. When opponent’s pieces are especially active on one side of the board, it is usually best to castle on the opposite side.
4. If you want to complicate the game you may consider this option. That may be true if you must play for a win due to a tournament situation, when the draw is not enough. Also that maybe done when you're playing against a stronger opponent, who is much better in simple/technical positions. That maybe your best bet.

You should not castle on the opposite sides when at least one of the following factors in true:

1. When you are behind in development and you need extra time to develop your pieces, it is usually not a good idea to give your opponent a straight forward way of launching an attack.
2. When the opponent’s pawns are advanced towards the side you’re about to castle, it is not a good idea to castle there (especially if the opponent’s king is castled on the opposite side). It will just give him a positional edge in the attack.
3. When there are open/semi-open files in-front of the side you’re about to castle, you should probably reconsider your decision to castle there (especially if your opponent has castled on the other side). That will give him more attacking possibilities, such as rook lifts, various sacrifices, doubling of pieces on the file, etc.
4. If you playing against a weaker opponent you may want to avoid castling opposite sides, in order to avoid sharp game and keep everything under control.

Note: These are general rules, not laws, meaning that there are always exceptions to them. When you’re making a decision what side to castle you should always take your time and evaluate all “pros” and “cons” and base your decision upon your own analysis. This is a very important decision. It pretty much dictates which way the game will continue. Take your time and think twice.

 I was editing my comment as you wrote, and I`m happy to see that my post-mortem included seeing how e4 and later capturing en passant was a disaster strategically speaking. a5 was also in my suggested  plan for candidate moves .  

You missed the powerful pawn push of ... d4!

Any time between move 7 and move 10 would've been a nice time to play that. It breaks open the centre and prepares to get your knight and bishop in to the action, which are currently quite blocked off.

Also, I really don't like the move 11... exf3??

The whole motivation behind having our pawn on e4 was to stunt white's development. Notice how white's LS bishop is very weak on g2, since it's trapped? Also notice how white is going to struggle to develop his pieces on the kingside? By capturing en passant, we're doing white a great service and making his life a whole lot easier. We don't want to give him this breathing space.

We could've tried something like 11... b5 instead, and just pawn stormed his queenside. Alternatively, 11... Ba3 looks quite good too, as this attempts to remove white's key defenders of his king, whilst maintaining a lock down on the kingside.

One of the main principles of chess is your board control.

The issue, often, with taking, is that you take with your advanced piece/pawn (thus losing it), and your opponent can come forward with his piece/pawn that was behind - as a result, your opponent *probably* gained more control in that exchange. 

This is the main reason why "keeping the tension" is such an important idea in chess - because either side initiating exchanges usually benefits the opponent. 

We can apply this idea: Let's take a look at the move ...d4, recommended right after white played 0-0-0. This move creates a powerful pawn center, and threatens ...d3 - as a result, white is almost forced to take, and this removes the e3 pawn, allowing your knight (or queen) to jump to ...d4 - this benefits you. This means that ...d4 likely is a good move, as long as you make sure e4 is safe (and it is).

You can note that this kind of maneuver is like a "positional tactic" - through a nearly forced short-term combination, you get a positional advantage out of it. 

I recommend three books on strategy, pick one that fits your time:

1. Quick study: 

Fred Wilson, Simple Attacking Plans – four straightforward principles demonstrated with 36 annotated games.

2. Longer Study: Yasser Seirawan, Winning Chess Strategies

3. Very Long Study:

Jeremy Silman, How to Reassess Your Chess, the 4^th Ed. (latest) is 658 pages and has so much excellent information it requires multiple reads, the old 1^st Ed. (about 200 pages) was required reading by the high school team county champion, 3^rd place state tournament team I coached (1^st and 2^nd place were loaded with Eastern European immigrants).

For endgames, either one of two books:

Jesus de la Villa, 100 Endgames You Must Know – stresses patterns rather than simple lists of moves or mechanical move sequences.

Jeremy Silman, Silman’s Complete Endgame Course from Beginnner to Master – tells you what you need to know based on your rating.

"... the 2000+ player for which 100 Endgames You Must Know is really intended ..."

https://web.archive.org/web/20140708105702/http://www.chesscafe.com/text/review645.pdf

"... Players from 2000 up to International Master will find 100 ENDGAMES YOU MUST KNOW quite useful." - IM John Donaldson

http://www.jeremysilman.com/shop/pc/100-Endgames-You-Must-Know-78p3863.htm

https://www.newinchess.com/media/wysiwyg/product_pdf/9026.pdf

"... I'm convinced that Silman's [Complete Endgame Course] will take its place in history as one of the most popular endgame books ever. It has already caught on with the average player in a big way, confirming Silman's status as the king of instructional writers. He writes in a clear and casual style, and time and again has shown the ability to reach those who feel intimidated by the lofty approach that a grandmaster will often take. ... Silman ... defines what he thinks is necessary to know at specific rating levels. For example, the beginner or unrated player needs to know ... Silman's idea is to wait until you climb in strength before you worry about more advanced material. Then, as a Class 'E' player (that's 1000-1199), one must learn ... Silman's book emphasizes to the student that the important thing is to master the strictly limited material at hand, rather than get confused by endings that won't help your results at that level. Perhaps even more importantly, Silman is able to use his teaching experience and talk to his readers in a way that they can handle, in a friendly manner and without condescension. ... I'll also repeat the point that David Ellinger in ChessCafe makes: '[This ...] demonstrates who this book will truly serve best: anybody who coaches chess. For me, as a perpetually near-2000 player who does part-time coaching, I’ve got in my hands a great resource that will have something for every student, no matter the rating.' ..." - IM John Watson (2007)

http://theweekinchess.com/john-watson-reviews/theres-an-end-to-it-all

https://web.archive.org/web/20140708103149/http://www.chesscafe.com/text/review594.pdf

https://www.silmanjamespress.com/shop/chess/how-to-reassess-your-chess-4th-edition/

"kindaspongey ... Thank you for your reccomendations. ..." - Giasira

Played this game on the weekend, I was unsure about move nr 18, when i castled with the plan of pushing the f-pawn. It worked out for me but mostly because opponent weakened squares unecessarily. Exchanging queens was also something I wasnt entirely sure of, though it was part of an overall plan to attack c3 and d4.

 I also wasn`t sure about move 10, taking with the c-pawn undoubles white`s pawns.. maybe I could have kept the tension here?

Rule #1 about chess: Generalizations are useful, but don't take them as absolutes. Other than the actual rules of chess, there are no "commandments" or principles that always work. 

What works in one case (e.g. having a bad plan is better than having no plan) may not work in another case (where just shuffling is the best you can do). 

That's why computers are able to crush humans - it proves that chess is really a game of concrete analysis and computation. 

Doesn't mean you can't improve, but it means that there are no real "magic bullets" to improve at chess - you just have to get a feel for when which principles work (intuition), and have good analysis skills (calculation and evalution) - these two skills take a lot of work to develop.