objective condition
Learning materials should be meaningful, easy to understand and not beyond your ability. Just like watching a good movie, the plot must be attractive and logical in order to make you enjoy it.
Subjective condition
You have to have a "small map" in your mind to help you connect new knowledge with old knowledge. You should have a "curious baby" heart, actively explore new knowledge and make good friends with existing knowledge. Don't forget to play an "interactive game" with the old knowledge and let them collide with each other! In this way, your brain map will be more perfect and new knowledge will become more meaningful!