For me, I don't count the number of hours I study or 'study for the sake of studying'. Rather, I set myself tasks such as- I will write notes for this subject and I will do x number of past papers because I know they will help me achieve higher marks. I know what my mark aim is and what I need to do to get it and I understand that I need to do these things by the end of the three weeks. However, I feel no pressure to constantly study 24/7 because, by breaking down the abstract notion of study and exam preparation into a discrete number of definite and well defined tasks, I feel more in control. I can study more efficiently as I understand what I am doing beforehand and why I am studying.
As others have mentioned before I really need to emphasize the importance of understanding instead of memorizing. This is not only because understanding allows you to apply concepts in exams but because it also helps you to memorize things (which are unfortunately necessary to acing exams). If you know the context behind the information you are memorizing, it is much easier to remember it. During the holidays I would focus on ensuring you have a full understanding of concepts whilst leaving the memorization to one to two weeks before the actual exam. I find that writing notes is a good way of consolidating information and ensuring you understand everything in the syllabus (another important point- FOLLOW THE SYLLABUS) whilst also providing an easy source of reference for when you start memorizing.
Past papers are also essential if you want to do well as they are a good way to make sure you really know the content and it gives you practice in applying your knowledge and understanding in answering questions in a clear and succinct way. Maths papers I generally do timed and closed book although for science subjects I often just do individual questions, sometimes open book, although I make sure I do a few papers under exam conditions right before the actual exam. English is also just doing a lot of reading tasks, again under timed conditions.
Well that's the system that works for me. I recommend you find the system that works for you. Ideally you would have already tested and refined this system during years 9-10 and prelims but I guess its not too late to look at what you've been doing and see how you could make it better. But whatever system you choose, make sure you know which tasks you're going to do. Set concrete goals for study such as doing past papers instead of abstract ones like 'study maths for 2 hours'. Ditch study timetables and instead set deadlines before which you aim to complete certain tasks (although give yourself ample time to avoid becoming stressed). Also, use the same system for similar subjects e.g. all sciences on the same system as some sort universality can help simplify things and make you more confident that you've studied enough for each subject.
Anyway, that's how I study, or at least how I plan to study during the holidays. I wish you the best of luck for your HSC but don't burn yourself out. Watch some TV, play some games, get some exercise, go out with friends (after all you need that practical experience for belonging as well
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