Underlining a sentence feels productive. Explaining the sentence without looking at it asks for something more. A pencil is useful because it creates a small space between what a book says and what you can explain in your own words.
Keep fewer, better notes
A note does not have to preserve everything. Choose the claim that matters, the example that makes it concrete, and the question that remains. A small number of thoughtful entries can be easier to revisit than pages of copied passages.
For technical material, add the assumptions behind an equation. For an argument, describe the evidence and a possible objection. The form of the note should match the kind of understanding you want to keep.
Keep the explanation you can use, not just the sentence you liked.
Use the idea immediately
Try a calculation, draw a diagram, or connect the argument to a practical situation. Using an idea exposes the parts that remain unclear. It also gives the note an example to return to later.
If the application fails, record why. A mistake can be more memorable than a correct answer, especially when you identify the step where the reasoning changed direction.
Return after a little time
Revisiting a note after a few days asks whether the explanation still stands on its own. Can you follow the steps? Are the symbols defined? Would one more example make the point clearer?
That is a manageable practice: read, explain, apply, then return. It keeps the notebook connected to learning rather than turning it into another thing to maintain for its own sake.
