1. Students are doing a lot of work - The work is being done by students. ( студенты делают много работы - работа делается студентами)
2.When we arrived they were pulling down the old building. -
The old building was being pulled down by they when we arrived.(Когда мы прибыли, они сбрасывали старое здание. - старое здание было сброшено ими когда мы прибыли)
3.Glinka was composed the opera “Ivan Susanin”. - The opera "Ivan Susanin" was being composed by Glinka. (Глинка написал оперу “Иван Сусанин”. - Опера "Иван Сусанин" была написала Глинкой).