During childhood, we just mug up and try to remember our subjects to pass our exams. But with maturity one realizes that study is to absorb things into the mind and try to understand the concepts so as to apply in future.
A guy who understands subjects most thoroughly can face any competitive exams in an easy way. But also one has to be able to remember or recall the subject in times of exams. So reading faster and understanding more is the actual technique to stand ahead of the competitors in studies.
Understanding takes time but is worth it. It is a tough task to understand some complex issues of physics, chemistry etc. but we can try to get over. Lack of understanding in these subjects will not help the mind become mature. The actual intention of education is mind maturity and understanding of subjects is the key to that maturity.
To have memory of any subject for a longtime the only way is to understand it to the fullest.
By questioning why: This is the technique to understand simple issues we come across. For example if one is using a bike or car, he operates many gears like first, top etc. Why one needs to shift gears is important to understand as per physics. Gear system decreases load on the engine and helps for better performance and longer mileage. This can be understood by just watching gear wheel and rear wheel of the bicycle.
By dig deep technique: This is one method to go to inner levels of any topic. This helps understand any complex issues. If going through tri-carboxylic cycle, you come across many intermediates of ATP formation indirectly like through NADH, FAD etc. One can just mug up to leave the issue. But if dug deep in you will see why NADH and FAD produces ATP during conversion from one form to other.
By trying to answer with self knowledge. If there is a concept to be understood, first try to give your opinion on it, then if it fails you can further enquire the actual reason for the concept.
By exploiting to a new application or use. There are some concepts which have certain ways or steps in the process. Though we understand them partially of why the steps or ways are present, we may not be satisfied for reasons like why they are necessary etc. Then the way to solve the quest is to apply it practically for any other purpose. This helps us know what steps are tough and why that intermediate is used to overcome that step etc.
By imagination. Some concepts can be understood better by visualization or imagination of process. This is required in case one has no access to suitable videos explaining the concept.
By comparing: Many scientific explanation are done by just comparing with other. Be it biology, physics, chemistry. Comparison gives differences between two concepts and reasons for why they are so.
By hearing to others view: Not all of us have same understanding pattern. Some understand in one way while other in other way. But, still everyone seems to have same conclusion. This understanding pattern depends on maturity of mind and the level of knowledge and exposure into the subject previously.
So when we feel a concept is partially understood then it is better to hear to others opinion on the said concept.