Jazz rhythm can be tricky for classical pianists to learn, because it can't be notated.
Let me put it another way: Jazz rhythm is actually pretty easy to learn, but since classical music is primarily learned through notation, classical pianists have to develop another way of learning if they want to play with a jazz rhythm.
A lot of non-jazz musicians try to play "jazzy" when they're learning jazz. But when you aim for "jazzy," the rhythm tends to be exaggerated and it sounds amateurish.
The way to sound jazzy is to listen to a lot of jazz and let it happen naturally. After all, your natural musicianship is a better learner than "thinking" about being jazzy.
Let's look at that last sentence again, because it's very important:
Your natural musicianship is a better learner than "thinking" about being jazzy.
Immerse yourself in jazz, through listening, jamming with other musicians, and going to live performances. And then don't try so hard to sound "jazzy." Don't worry: you'll gradually start playing more and more in the rhythmic language of jazz. Seen another way, you have no choice. Jazz rhythm can't be rushed any more than a slow-simmered soup can be cooked fast. The flavors need time to blend.
Put in the time, and trust your innate musicality. Above all, enjoy the journey and "let the music flow!"
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