Using conditional 2 or 3. 1. the environment ( much cleaner if people (not so many chemicals. 2. i ( if i ( more time. 3. if people (not so much litter, our streets ( so tired. 4. i ( to the cinema tonight if i (not so tired. 5. if i ( you were ill, i ( you. 6. she (not if it (not that important.

1. would be, didn't use
2. would help, had 
3. didn't drop, would be ( здесь, похоже, неправильно задание дано. Должно быть  much cleaner. )
4. would, weren't
5. had known, would have visited
6. wouldn't have disturbed, hadn't been