Here are the plural forms of the nouns listed:

1. person - persons / people
2. woman - women
3. milk - (uncountable, no plural form)
4. knife - knives
5. child - children
6. foot - feet
7. sheep - sheep (no change)
8. luggage - (uncountable, no plural form)
9. water - (uncountable, no plural form)
10. tooth - teeth
11. student - students
12. fish - fish (or fishes, depending on context)
13. key - keys
14. information - (uncountable, no plural form)
15. man - men
16. housewife - housewives
17. goose - geese
18. piano - pianos
19. sugar - (uncountable, no plural form)
20. box - boxes