3) People built a bridge over the river. 4) Peter finished the composition. 5) A lot of people planted these flowers. ПРЕВРАТИТЕ ЭТО В PASIVE
A bridge (over the river) was built by the people (не уверена насчёт the) The composition was finished by Peter These flowers were being planted by a lot of people
3) ‘the’ не нужно, можно и без people) 5) these flowers were planted by a lot of people
спасибо за поправки)
Пожалуйста, сам не без греха :)