Поставьте множ. число boy, toy, day, spy, tree, book, thief, half, snowman, pilot, roof, cliff, mouse, child, tooth, fly, bus, bush, gnome, stone, rose, kilo, video, audio, mosquito.

Boys, toys, days, spies, trees, books, thieves, halves, snowmen, pilots, roofs, cliffs, mice, children, teeth, flies, busses, bushes, gnomes, stones, roses, kilos, videos, audios, mosquitoes.

1. Have they just fastened the safety belts? What have they just fastened? Why have they just fastened the safety belts? Who has just fastened the safety belts?

2. Does Susan usually travel by bus? How does Susan usually travel? Where does Susan usually travel by bus? Who usually travels by bus?

3. Did her friends travel by bus last year? When did her friends travel by bus? How did her friends travel last year? Who travelled by bus last year?

4. Will David go to Turkey by ship? How will David go to Turkey? Where will David go by ship? Who will go to Turkey by ship?

2) They are still excavating the site of China’s northern province.

The site of China’s northern province is still being excavated

3)) They'll not deliver the parcel tomorrow.

The parcel will not be delivered tomorrow

4) She has cancelled this meeting.

The meeting has been cancelled by her

5) He stole my wallet.

My wallet was stolen by him

6) T. Shevchenko painted “A peasant family”.

“A peasant family” was painted by T.Shevchenko

7) We encourage visitors to our heritage sites.

Visitors are encouraged to the heritage sites

8) They completed the monument in 1648.

The monument was completed in 1648

9) Four domed chambers surround the Taj.

Taj is surrounded by four domed chambers

10) Twenty thousand workers built the Taj Mahal entirely of white marble

Taj Mahal was build entirely of white marble (by twenty thousand workers)

Дополнение: "by them" писать бессмысленно, не важно кто это - они. Для этого и полезен passive