1. I've finished all of the Cambridge books. What should I do next?
My advice is to do them again! This time, don't use the books for testing. Study and analyse the questions and answers carefully, use a dictionary to look up new vocabulary, try writing essays without a time limit, and record yourself answering the speaking questions. There is so much that you can learn from past exam papers.
2. Which is correct: 'sport' or 'sports'?
Both are correct. 'Sport' is uncountable and refers to all sports in general (e.g. I like sport). 'Sports' is the countable noun and refers to more than one individual sport (e.g. I like team sports).
3. How would you define 'majority', 'few' and 'many' in percentages?
'Majority' normally means more than 50%. 'Few' and 'many' are normally used with numbers rather than percentages: a small percentage of people could still refer to a large number of (many) people. 'Many' and 'few' are difficult to define because it depends on the context, but in general they refer to large and small numbers respectively.
4. Is it correct to write "water is evaporated"?
Yes, you can write "water is evaporated by the sun", but I would probably just write "water evaporates".
5. How many points should I mention in a task 2 essay?
There is no rule about how many points or ideas you need. It's possible to write a full paragraph about just one idea, reason or argument. I tend to write between one and three ideas per paragraph. This lesson explains how I do it.