Need help on bettering exam technique

I'm going into 3rd year at a Scottish Uni (4 years for a BSc) and in the first two years I have been struggling to get good grades for my physics courses. I have been getting C's and D's despite reading all the lecture notes, attending lectures and taking extra notes there, reading a couple of useful textbooks for each module and doing most of the past paper questions. I have a solid grasp on the material being covered in lectures. But I'm not sure if I should do a couple of past papers and hand it in for the lecturer to mark (is this allowed?). So far I've only been writing out the formulas and calculation workings in the exam. How should I improve on this to get more marks? Should I write explanations before and after the calculation? How can I show that I've done extra reading in exams?

Next year, I'll be studying Quantum Mechanics, Energy and Matter, Electricity and Magnetism. How can I do better in theory-based exams as well?

Any useful study guides for science?*

have you tried seeing your tutor and explaining that you don't understand where you're dropping marks in the exams?

maybe they've got model answers you could compare your scripts to or as you suggest taking in some completed past papers.

No, but I think I'll do that. They didn't publish the answers to the past papers so I was a bit apprehensive on taking in completed past papers. When can students start handing in past papers? Is mid-October too early?