3.your friend's umbrella

4.OK

5. Charles'/Charles's daughter

6. Helen and Dan's son

7.OK

8.last Monday's newspaper

9.OK

10.OK

11.your children's friends

12.our neighbours' garden

13.OK

14. David's hair

15.Katherine's party

16.OK

17.parents of Mike's car

18.OK

19.government's economic policy