1. We didn't feel sad. We were happy. - нужна первая форма
2. What are you doing? - I am looking for my cap. I can`t find it. - искать
3. "No", said the king. He said it many times. - много раз (исчисляемое)- устойчивое выражение look for
4. Mrs Jackson`s sons want to be teachers. - все верно
5. What nice food! Your sister is a wonderful cook, isn`t she? - неисчисляемое существительное, не нужен артикль
6. He`s washed his car, hasn`t he? - краткая форма - he has
7. Money don`t grow in the trees. - в словосочетнии "на деревьях" имеется ввиду "внутри веток" поэтому предлог in
8. Did we have to buy much bread. - неисчисляемое существительное
9. Do you go to school by bus? - устойчивое выражение - без артикля
10. Don't I understand the last word in the text? - именно последнее слово - поэтому определенный артикль.