Here are the combined sentences:

1. Her father has just returned from Paris.
2. Bill, whom we saw at the dance, wants to go out with me.
3. Flies, which mostly come in the summer, carry disease.
4. Swimming is a good sport because it makes people strong.
5. Grass is always green in England, which cows and horses love.
6. Air, which we breathe, is made up of many gases.
7. Jackie Chan, a famous actor, has a sister whom I know well.
8. The house we are living in needs mending.
9. This tree, whose branches are dead, should be cut down.
10. Tell me the reason you left the city for.
11. That river is not safe for swimming because its water is polluted.
12. Can you suggest the time when we'll meet?
13. Mr. Johnson, whose daughter has been living in Spain for 4 years, is going there.