Here are the combined sentences:

1. **Her father, who has just returned from Paris, has been to Paris.**

2. **Bill, whom we saw at the dance, wants to go out with me.**

3. **Flies, which carry disease, come mostly in the summer.**

4. **Swimming, which is a good sport, makes people strong.**

5. **Grass, which is always green in England, is loved by cows and horses.**