The key answer of exercise 1

a)

1. ✓
2. he might forget
3. she has never been
4. ✓
5. you had to stay in to study
6. he was doing his homework
7. ✓
8. she had written
9. you wouldn't be late
10. ✓

b)

1. we could stay there until the end of the month
2. he'd been busy the week before
3. they's booked a flight for the next month / the following month
4. if he had to go then
5. he was working late that night / evening
6. I didn't like that restaurant
7. I hadn't called her the day before / the previous day
8. if he was coming to dinner the next day / the following day

c)

1. to go to bed
2. to give her my phone number
3. where I had learned to speak English
4. if we understood the instructions
5. not to call her at work
6. what time I would be home
7. to make less noise
8. not to forget his doctor's appointment