Correct the mistakes:
1. they were late but the lecture didn’t start yet.
2. i had remembered that i left the iron switched on.
3. the floor was slippery because tom spilt some juice on it.
4. mary had given me the book i hadn’t read before.
5. he returned home because he didn’t lock the door.
6. before the pupils had started the exam, they had switched their mobiles off.