3. Tomatoes should be planted (by people) in the spring.
4. This report must be signed by Mr. Hook.
5. A new post office on First Street may be built by someone.
6. Beer may not be sold (by people) to minors.
7. I can be reached (by people) at 555-3815.
8. Stamps have to be placed (by people) in the upper right-hand corner of an envelope.
9. That fence should be painted by someone. | Ought that fence to be painted by someone.
10. The weather cannot be controlled (by people).
11. Our car had to be fixed (by someone) before we left for Chicago.
12. The assignment must be done by all of the student.
В скобках части предложения, которые в принципе можно опустить (и я бы опустила, для благозвучности), но если Вы считаете нужным - оставьте их.
Предложение 9 имеет два возможных варианта ответа, так как глагол ought to в наше время используется очень редко.