p. 166 ex. 3. 1 — a; 2 — b; 3 — a; 4 — b; 5 — c; 6 — c; 7 — c; 8 — c.
p. 166 ex. 4. 1. When I opened the door, the lesson had begun and the teacher was explaining something to the class.
2. I was cleaning the carpet, when the dog came and shook himself.
3. When the bell rang, the children ran to the classroom and took their seats.
4. We had a good holiday after we had done all the work.
5. Before we took the boy to the theatre, he had never seen a play.
6. By the time we arrived, the party had finished.
7. When I got to the playground my friend had already left so I didn’t see him that day.
8. What were you doing, when I called you? I was cooking.