Перевод предложений из активного залога в пассивный требует перестановки подлежащего и дополнения, а также изменения формы глагола. Давайте изменим каждое из предложений:
You should take these tablets before meals.
- These tablets should be taken before meals.
You must wash coloured clothes separately.
- Coloured clothes must be washed separately.
The teacher sent him out of the classroom.
- He was sent out of the classroom by the teacher.
Thousands of British tourists will visit Spain this summer.
- Spain will be visited by thousands of British tourists this summer.
The dentist pulled out my rotten tooth.
- My rotten tooth was pulled out by the dentist.
The police are questioning him now.
- He is being questioned by the police now.
Someone has made a complaint.
- A complaint has been made.
They had left the lights on.
- The lights had been left on.
I broke my arm when I fell out of a tree.
- My arm was broken when I fell out of a tree.
The horrible old man was slowly poisoning the cat.
- The cat was being slowly poisoned by the horrible old man.
The snow will have covered the mountains by Christmas.
- The mountains will have been covered by the snow by Christmas.
The waitress serves breakfast at 7 am.
- Breakfast is served by the waitress at 7 am.
Who delivered the parcel?
- By whom was the parcel delivered?
Her parents made her clean her room.
- She was made to clean her room by her parents.
They will have finished the work by tonight.
- The work will have been finished by tonight.
Someone had warned her that she might lose her job.
- She had been warned that she might lose her job.
Who discovered America?
- By whom was America discovered?
Who is going to feed your dog?
- By whom is your dog going to be fed?
Who answered the phone?
- By whom was the phone answered?
Who did they give the prize to?
- Who was the prize given to? (или) To whom was the prize given?
Which building are they going to knock down?
- Which building is going to be knocked down by them?
Обратите внимание, что в некоторых случаях пассивная форма может звучать несколько искусственно, особенно в вопросах. Однако, это правильные преобразования с сохранением структуры пассивного залога.