Why theory? Memorisation. Instead of trying to memorise a long series of notes, trained musicians remember a shorter series of larger patterns. They understand how the various patterns related to each other. It is important to understand that notes don't move at random. Each style of music has conventions that composers understand and work with.
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I could go on.
Now ear training. Ultimately we all play be ear. Training your ear will shorten the time it takes to get there. You don't need perfect pitch but you do need good relative pitch. You need to be able to hear nuance in the professional performances you are listening to.
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Remember I give online lessons, now is four countries.
You've got to learn your instrument. Then, you practice, practice, practice. And then, when you finally get up there on the bandstand, forget all that and just wail.
I'm a professional pianist and music educator in West Toronto Ontario. I'm also a devoted student and teacher of the drums.