4. Angelina Jolie began taking an interest in charity work. She was filming in Cambodia then.

Rewritten: Angelina Jolie began taking an interest in charity work while she was filming in Cambodia.
5. While I was walking along the street, I noticed a homeless boy sleeping in the cardboard box.

Rewritten: I noticed a homeless boy sleeping in the cardboard box when I was walking along the street.
6. John began playing the piano 10 years ago.

Rewritten: John has been playing the piano for 10 years.
7. They last visited me five years ago.

Rewritten: They haven't visited me for five years.
8. May I borrow your bicycle?

Rewritten: Would you mind if I borrowed your bicycle?
9. The students have discussed the pollution problems since last week.

Rewritten: The pollution problems have been discussed by the students since last week.