Key words are the thing to look for and what they are asking you to do. If a question asks you to explain, then do so; ensure there is a 'because' somewhere in the answer.
If the question asks you to describe, do so. For instance, describe how a meander forms will be different from explain how a meander forms.
There is always a clue in the question, and he needs to look for it.
If a question is asking for a concrete example of something - give it.
If you are asked to give two reasons for something, do so.
I always advised my students to take 5 minutes to read the exam paper, decide what they were going to answer and underline the key words in each question, like explain and describe, outline, give reasons etc. Also, to note little words that they may miss if reading the question fast - a not makes a huge difference to a question.
When I was marking one year, an A* candidate ballsed up all the 8 mark (explain) questions by not putting 'because' in, and her answers didn't explain, they described. This lost her 30 marks over the paper.
I am a senior examiner now, and I am still giving the same advice to ds in Year 10.