Ответ:

2)Had been studying

3) had never seen

4) had been snowing

5) had been cooking

6) had come

7)had finished

8)had been collecting

Объяснение:

past perfect - действие закончено в прошлом

past perfect continuous - действие закончено в прошлом, НО уделяется внимание на то, сколько длилось данное действие.

Past Perfect - действие закончилось к какому -то времени в прошлом

Past perfect continious - действие длилось в прошлом, и было прервано к какому то времени.

Их можно легко отличить с наречиями "For" и "by". Если в предложении есть Time expression "For", то всегда - Past perfect continious. Если "by" - past perfect

2. i had been studiying art for years before i began to paint

3. she had never seen Picasso's works until she went to university

4. it was a cold day it had been snowing for about twelve hours

5. she had been cooking the cake for several hours and it was very tasty

6. they had came to the museum before the excursion started

7. by the time she entered university, she had finished an art course

8. he had been collecting coins for many years before he presented them to his grandchildren