How do I approach this question? Our teacher never explained it in class!

Any question in physics will boil down to a simple conservation law, even at the relatively difficult A level exams.

Basically, in order to successfully solve this question, you need to identify 3 key things:

1. Is there a net force acting on the object?

2. Is there a change/ net transfer of energy?

3. Does this question involve waves?

You will eventually see that the 1) and 2) essentially boil down to the same question - Will the object at time t₂ be in the same state as the object at time t₁? (Here by state i mean any measurement - acceleration, or velocity are the two most common ones at A level, though things like charge, capacitance or angular velocity).

This train of thought will allow you to solve at least 60% of the questions on the paper - the remaining 40% are about experiments, and those are all about memorisation, unfortunately.