1.The film has been on for half an hour.(应该film单数吧)
2.The film has been on since half an hour ago.
3.It is half an hour since the film began.
1.He has kept the book for 2 months.
2.He has kept the book since 2 months ago.
3.It is two months since he borrowed the book.
1.We have lived in Shenzhen for 10 years.
2.We have lived in Shenzhen since 10 years ago.
3.It is 10 years since we lived in Shenzhen.