1. mark is too young to see the horror film
⇒mark is not old enough to see the horror film.
3. They once started these machines by hand.
⇒They used to start these machines by hand.
5.She was sleeping and the fire started
⇒ The fire started while she was sleeping.
2.They should quit smoking.
⇒ They had better quit smoking.
4.Mary went shopping because she wanted to buy some jeans.
⇒ Mary went shopping in order to buy some jeans.