Answer:
1. I went to bed early because I was tired.
2. We went for a walk by the sea since it was a nice day.
3. We came in very quietly since we didn't want to wake anybody up.
4. She was late for work because she missed the bus.
5. We didn't know what time it was because none of us had a watch.
6. We didn't recognize her as we hadn't seen Lisa for a long time.
7. Archi was very fat because he ate too much.
8. I walked in as the door was open.
9. We watched television all evening as we had nothing better to do.
10. We see them quite often since they live near us.