The key answer of exercise 6

a)

1. Did you use to spend your holidays by the sea?
2. We didn't use to have a dog when I was a child.
3. I used to love reading in the evening, but now I'm too tired.
4. I remember how I used to sit in my grandfather's studio and watch him paint.
5. Children always used to play around on the streets in the old days, but there's nobody here now.
6. They used to live in a big house, but they had to move.

b)

Sentences 1, 4, 5