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Is chess knowledge and playing different things? Do I need to practise?
Yes, then analyze your games to find where your knowledge and application failed to meet or where your knowledge or ability may be lacking. Then try to fix that problem in future games. Rinse and repeat.
It is also good to practice in longer time control games.
Yes. Chess knowledge and playing are different things. And yes, most experts recommend practice. You can be a whiz at memorizing openings, but eventually openings lead to middlegames where more general knowledge of chess is required like coming up with a plan. Waiting for your opponent to blunder, strangely enough, is not considered high level planning. Oh, and yes, you do have to review your games to get better. Endgames and pawn play, the boring sh#t; that's where you grind out improvement...IMHO 
It's going to benefit you more if you don't bother learning opening lines and concentrate more on opening principles. Chess Mentor has some good lessons on this.
It's going to benefit you more if you don't bother learning opening lines and concentrate more on opening principles. Chess Mentor has some good lessons on this.
https://www.chess.com/article/view/the-principles-of-the-opening
This?
It's going to benefit you more if you don't bother learning opening lines and concentrate more on opening principles. Chess Mentor has some good lessons on this.
https://www.chess.com/article/view/the-principles-of-the-opening
This?
Damn fine start...When you have those down, try these to see how they apply to real situations:
https://www.chess.com/blog/webmaster/free-chess-mentor-courses
I'm finding it easy learning openings and tactics but another using it there and then. Why is this? The pressure? Why am I finding it another thing using everything on the spot in terms of playing?
Is chess knowledge and playing different things? Do I need to practise?