What do you think of as the fundamentals in poker? The essential pieces that build a foundation for being able to succeed at the game? First I think it's important to break up fundamentals into two categories: skills and knowledge.
Skills are actions trained through repetitive practice, most effectively learned when your practice replicates game-like scenarios. In a sport like basketball, some skills are developed at a young age and practiced over time, while others are trained for a specific purpose under a specific coach.
The above list is truly fundamental to the sport - these are the skills that are learned at the very beginning, but also the skills that one never stops practicing. You can *always* improve a little bit more, and this type of training will continue to pay dividends on the court. What might this list look like in poker?
I struggled a bit to work backward and identify the core skills in poker (for example betting rather than continuation betting), but I believe these are the basic elements. It feels funny to consider that we need to train our checking - we don't normally describe poker players as being excellent 'folders' the same way you might say a basketball player is an elite passer. You might find it easier to consider that you need to train for checking turn OOP after check/raising the flop and facing a call. This added context gives you a game-realistic scenario in which to practice the skill of checking.
Knowledge, however, is typically conveyed in a classroom setting, with different teaching methods being most effective for different learning styles. Professional players and enthusiastic fans alike can develop fundamental knowledge by consuming content and dedicating effort to understanding the strategy of a game they love.
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Poker is complex and multifaceted, meaning there is a wide range of knowledge I consider fundamental to the game. This is surely an incomplete list but gives an idea of the topics necessary to build foundational knowledge.
It's important to recognize that theoretical knowledge does not by itself make a good poker player. We must both understand the purpose of a play and develop experience running a play before claiming mastery. Too often as poker players, we rely only on understanding and underestimate the importance of practice, presumably because we play a mental sport. I believe this severely holds players back in their development - and I know this from personal experience.
In order to translate your knowledge of the game into an actionable strategy, you need to put in repetitions, either with a trainer or in a live game. Until you've shown an ability to execute at the level of a professional player on the felt, your game is no better or worse than that of the educated fan, watching from the sidelines.