1. The United Nations has (ООН в единственном числе) meetings where world leaders agree on what to do about global warming. If there is too much pollution in the air, some people have (conditional - как факт.Если будет 1-ое условие, то точно произойдет 2-ое) health problems.
2. Ecologists looked for special spots on white pine trees to know if there
were(согласование времен, множественное число) high levels of ozone.
3.Many types of air pollution are invisible, but some can be seen(пассивный залог) with the naked eye.
4. The teacher asked the boys if they had done(согласование времен в косвенной речи) the task.
5.Satellite images proved that deforestation had increased (согласование времен) rapidly.
6.The passage is so difficult that I can't (по смыслу) understand it.
7.Experts said that less than 3 per cent of the water on Earth was(согласование) fresh water.
8.His health has improved since he returned (грамматические правила) from the hills.
9.If we had sorted the waste, we would have saved (3 conditional) millions of kilowatts of energy.
2. I’ve tried.
3. I’ve been hiking, rafting and horse-riding. (Это скорее present perfect continuous)
4. I have also learned about painting and video game design.
5. The coolest thing I’ve learned so far is how to develop survival skills.
6. I’ve taken some fantastic photos.
7. The camp leaders and some of the other campers have asked me to email some to them.
8. The view from the top of the mountain is the most breathtaking sight I’ve ever seen.
9. I haven’t made a tree house yet.
10. I haven’t gone fishing yet but I have had a fantastic picnic by the lake.
Предложения, в которых вы можете встретить have + V3, написаны в Present Perfect.