There's a lot that goes into teaching somebody a game or skill that's completely new to them. And there's also a reason why people who make excellent players don't always make very good teachers, and vice-versa. This isn't always true, and it's definitely less true in certain fields and games than in others, but it is definitely a trend.
The main issue comes down to extrinsic knowledge and intrinsic knowledge, which is another way of saying, "stuff you know that you know" and "stuff you know without thinking about." In more fast-paced games with quick, snap-decisions, a lot of it comes down to intrinsic knowledge. You want to make decisions instantly without thinking. You "feel" the right choice, because you don't have time to logically map stuff out. The more fast-paced your game, the more important this is. The more slow-paced, the more you can analyze and break things down. You need both to be a successful player, but the extent depends on the game.
Good teachers have lots of extrinsic knowledge. They can explain the things that work, and why they work. There's a surprising number of players I've seen who, when asked what a newer player can do to see better results, reply with "play better." Which, while technically accurate, isn't always the most helpful.*
A more helpful answer might be, "you aren't paying enough attention to this enough, and it's causing you to make poor decisions." Or, "you need to widen your stance, so you don't lose balance in this situation."
Compare that to really intuitive players. Though they can definitely be successful, they typically don't teach quite as well. "I don't know, I just kind of do it," is the kind of thing you hear from them.
This is why even if your training partner is the best player in the world, it's possible they won't be able to help you except by being an inspiration. But if you have a knowledgeable and experienced player teaching you, one that understands improvement and skill development (even though they may not be a high-ranking player), you might become an amazing player with that person as your guide.
Not all knowledgeable people with high levels of extrinsic knowledge are good teachers. Good teachers also know how to transform it into intrinsic knowledge in somebody else. They know how to drill new players, they understand which skills should be learned first and what to prioritize. They can help you unlearn bad habits and how to avoid them. Just being a fact dispensary and trivia master isn't enough. Teachers can't just possess, they must transfer.
And we haven't even touched on the personal aspect of teaching! Personalities need to mesh. Your student needs to trust not just your knowledge, but your character. You need to do little things that maintain their interest--you can't turn a completely unwilling student into a great one, but there ARE little things that can turn material that's hard to digest into stuff that's fun and engaging. And even if you can't take a bad student and turn them into a great one, there are lots of little things that can take an agreeable and enthusiastic student and make them unhappy and closed off. When your student is amazing, just avoiding lots of little mistakes as a teacher is even more important than being an amazing empath.
It can also be very hard to be a teacher and a player at the same time; the time you spend teaching is time you could spend practicing. Teachers are also expected to have more knowledge (and when the student is very new, more skill) so it can be tough to try and play your hardest and push yourself while you're taking it easy on someone new. If you completely stomp a new player too much and don't give them the chance to try the things you teach them, then you're wasting both of your time (and probably discouraging them).
A decent way to mitigate that problem is to set up drills that both of you can work on. If you're a higher level player than the other person, it sets a solid standard for them to work to. If it's an important fundamental, then there is never a reason not to be drilling it, no matter how good you are.
Thanks for reading! See you next week.
*"Play better" isn't very helpful.... except when it is. Sometimes players look for gimmicks and secret answers that will immediately solve all their problems, when really, the answer is just "train and improve the things you're already doing." When you avoid the trap of magic bullet hunting and strive to improve your fundamentals, you move to becoming a solid and amazing player. Sometimes, just hearing that from a pro is what it takes to get better.