I.

1. Because factories produce waste and pour it into rivers and lakes, many rivers and lakes are poisoned.

Because of the production waste from factories, many rivers and lakes are poisoned.

2. They provided food, then cleared up the debris.

=>After they provided food, they cleared up the debris.

3. The local people have to use water from the polluted river. They don't have enough fresh water.

=>If the local people had enough fresh water, they wouldn't have to use water from the polluted river.

4. I'm not you, but I think you should recycle these plastic carrier bags.

=>If I were you, I should recycle these plastic carrier bags.

5. All flights had to be canceled as a result of the pilots' sudden strike. 

=>The pilot suddenly struck, so all flights had to be canceled.

6. You have to keep quiet if you want to stay here.

=>She told me that I had to keep quiet if I wanted to stay there.

II,

1. Water pollution is the contamination of bodies of water such as lakes, rivers, oceans, and groundwater.

2. Have American celebrated Thanksgiving Day on the Fourth Thursday of November since 1864?

3. Scotland is famous for its rich culture as well as its amazing natural beauty.

4. The Statue of Liberty in New York is a monument which symbolizes freedom.