Ответ:
1. We didn't recognize Jane because she had cut her hair.
2. My sister called me to tell me I had left my school bag at home.
3. When Sam turned on the TV, the concert had finished.
4. Peter didn't lend me the book because he hadn't finished it yet.
5. My friend had failed all his exams because he didn't study hard.
6. When they got to the hotel, they saw that somebody had broken the window.