Разберем каждое предложение и преобразуем его в пассивный залог.
1. My mum bought a cake yesterday.
A cake was bought by my mum yesterday.
Торт был куплен моей мамой вчера.
2. He has painted the fence.
The fence has been painted by him.
Забор был покрашен им.
3. The police arrested the thieves.
The thieves were arrested by the police.
Воры были арестованы полицией.
4. My sister invited some friends.
Some friends were invited by my sister.
Некоторые друзья были приглашены моей сестрой.
5. She sent the parcel yesterday.
The parcel was sent by her yesterday.
Посылка была отправлена ею вчера.
6. John received the money last week.
The money was received by John last week.
Деньги были получены Джоном на прошлой неделе.
7. They serve dinner at seven.
Dinner is served at seven.
Ужин подается в семь.
8. The girl will tell the truth tomorrow.
The truth will be told by the girl tomorrow.
Правда будет сказана девочкой завтра.
Ответ: см.выше