If you have a concrete objective (speaking a language, passing an exam), how you practice should match the intended use.
An extension of this idea is that learning broadly is a bad idea - that you won't remember "useless" knowledge. But this is false. Having an extensive knowledge base makes learning easier.

Directness in learning

In improving your knowledge base, you're not optimising for a specific goal, but all future learning goals.
<ul>
<li>Read more textbooks and less popular books.</li>
<li>Take more online classes on fundamental topics.</li>
<li>When you can't build higher, go deeper. You may be hitting the ceiling with your improvement in a specific area. The only way to get better is to do more foundational work.</li>
</ul>

Foundations you should build

The magical "intuition" for hard subjects we notice in people like Albert Einstein and Richard Feynman is owed to their extensive knowledge base they could draw from.
The broader and more varied the situations you need to perform in, the broader your knowledge base should be to help you think better.

Many goals in learning require transfer

Direct practice is not the opposite of deep understanding.
A naive approach to mastering, for example, physics problems, is to continue practising exam questions. But practising limited exam questions is not the same as the range of problems you will find in the real world. To solve real problems, you need a deep structure for understanding the problems, not just memorised solution patterns that you cannot apply.

Broader skills benefit more from theory

We tend to think of skills reasonably broadly, but our skills are very specific.
Direct learning minimizes the chance that we will focus on learning information unrelated to our actual goal.

Direct learning works for a particular goal

"Useless" knowledge can be quite valuable. Everything you want to learn depends on the foundations you've built beneath them.

Broader skills benefit more from theory

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How to build general skills

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