this can actually be kind of confusing to some people who already practice lucid dreaming.
there are two kinds of learning
learning through experience
and being taught
while practicing something that doesnt require any form of previous knowledge, like skateboarding, surfing, playing guitar, etc., you can learn simply by practicing.
this form of learning you can do in a lucid dream.
the only issue with this is that it does not contribute to any form of muscle gain, considering your already asleep.
meaning that you CAN practice martial arts in lucid dreams, as long as you already know the form you want to practice, or your creating a new one.
the best part is, if you have good dream recall, you can copy the form in your notebook and memorize it later, allowing you to get in double the practice you normally would spend trying to create a new form.
being taught is simply learning something from someone else, something being passed down, like a math formula, a specific technique, a recipe, anything that requires previous knowledge.
in certain circumstances, you actually can learn some of these in a dream, but only because your mind fabricates it for you to find.
like something in a book you might happen to find lying around in your dream.
its pretty much like finding something you’ve already created, you just dont know what it is, but like i said, this is rare, and im just saying it has the possibility to happen.