An oprtator has to shoot a film. - An operator must shoot a film.
A veterinarian has to help animals with their health problems - A veterinarian must help animals with their health problems.
A doctor has to help people with their health problems. A doctor must to help people with their health problems. - Можно составить и так и такю Но have to , has to имеет значение необходимо , а must выражает большее должествование , т.е. надо, необходимо, если нужно выбрать только один вариант для предложения , то в первом, наверное has to - необходимо снять фильм, но может и не снимать , а во втором must т.к. ветеринар и доктор должны, обязаны животным, людям с проблемами здоровья . - Предлагаю такое решение
1) Tom returned from the cinema at five o'clock.
2) Tom had returned from the cinema by five o'clock.
3) I finished my homework at seven o'clock.
4) I had finished my homework by seven o'clock.
5) He thought that he had lost the money.
6) Ann told me that she had seen an interesting film.
7) When I came home, mother had already cooked dinner.
8) When father returned from work we had already done our homework.
9) When the teacher entered the classroom, the pupils had already opened their books.
10) Kate gave me the book which she had bought the day before.
11) Nick showed the teacher the picture which he had drawn.
12) The boy gave the goats the grass which he had brought from the field.
13) Mother saw that Nick hadn't wash his hands.
14) The teacher understood that Lena hadn't done her homework.
15) I knew that my friend hadn't come yet.
16) When I woke up yesterday, father had already gone to work.
17) Nick thought that his father hadn't come home yet.
18) Mary told us that she had cooked a good dinner.
19) Yesterday found the book which I had lost in summer.
20) When we came to the station, the train had already left.
Объяснение:
Самое главное - это изучить правила постановки глаголов в Past Simple и Past Perfect. Затем перевести каждое предложение и понять его смысл. Ну и в процессе написания не забыть про неправильные формы глаголов, которые сушествуют в английском языке.