Ответ:

1. Add -s: helmet, toy, handbag, guitar, boy, cap

2. Add -es: watch, box, bus, hero

3. Change y → ies: baby, lady

4. Change f/fe → ves: scarf, knife

5. Irregular plurals: child

Ответ: helmet, toy, handbag, guitar, boy, cap; watches, boxes, buses, heroes; babies, ladies; scarves, knives; children.