When reading to learn, also called study reading, it is never enough to sit back with your reading material, move your eyes across page after page until you’ve reached the end of your assignment, and expect to remember what you just read, let alone actually learn what you needed to or were expected to from the reading. Therefore, you need to be an active reader.
My preferred method of active reading has always been to write down what I think is important as I read. I have terrible memory retention when just reading, but I was always told that writing down what you learn as you read uses two different parts of your brain, and thus can help you memorize things better. It also never hurts to have notes to look back on if you find yourself struggling.