Here are the corrections for each sentence:

1. She wishes there **were** enough food for everyone in the world.
2. Carson is very depressing as his closest friend is moving to **another** town soon.
3. Nick told Jessica if she had gone to Nancy's birthday party the previous night. (**Correction**: "Nick told Jessica **that** she had gone...")
4. Of the two sisters, Marie is the most intelligent and she does well at school. (**Correction**: "at" should be "in")
5. I am confident that life in the country has improved **significantly** in the last five years.
6. We wish we could consult an expert on how to solve this problem. (**Correction**: "can" should be "could")
7. The environmentalists suggested to plant more trees to improve the city's landscape and the environment. (**Correction**: "to plant" should be "planting" and "to improve" should be "improving")
8. He worked **much harder** for his exams than he did for his everyday school work.

Corrected sentences:

1. She wishes there were enough food for everyone in the world.
2. Carson is very depressing as his closest friend is moving to another town soon.
3. Nick told Jessica that she had gone to Nancy's birthday party the previous night.
4. Of the two sisters, Marie is the most intelligent and she does well in school.
5. I am confident that life in the country has improved significantly in the last five years.
6. We wish we could consult an expert on how to solve this problem.
7. The environmentalists suggested planting more trees to improve the city's landscape and the environment.
8. He worked much harder for his exams than he did for his everyday school work.