2. No "go to". Where do they go every day.
4. "is going" - would be better. in te first and second. The second sentence would be better than the first as it has a redundancey in it.
1,3, 5 and 6 are just fine.
1. should be "after studying in NZ for five years."
1. should be: "Who never had any problems..."
1. Should read "Have you ever had problems with the water?" or, WHich problems haven't you had with the water?
2. "haven't" shouldn't be capitalized.
3. Which language are you perfect in?
3. Which language must you still practice?
5. "because YOU think YOU already know everything.
5. How MANY of you...because YOU...YOU...
5. How often DO some of you take notes?
Why do some of you never take notes. (the 'any' is redundant)
6. ..........whenever he wants to go. (would be better).
"Where will they take him?" or "Where will they take him whenever he's ready to go?"
With regard to the "He/him"/"Who/whom"? Probably because most don't take the time to write correctly. I'm sure it's even worse in German as things are multiplied by 3 (male, female, neuter).