c) Past Simple или Past Continuous.
18. When he (to paint) the wall, he (to use) scaffolding.
19. When I (to sit) in the office, I suddenly (to see) my old friend.
20. When Nick (to draft) a plan, his computer (to crash).
9. I (to sign) the contract when my boss (to open) the door.
d) Past Simple или Past Perfect.
When they (to enter) the hall, the conference already (to begin).
The teacher (to understand) that Helen (not to do) her homework.
He (to think) that he (to lose) the money.
Leo (to say) that he (to get) his education in Cape Town.
By the time we (to come) to see him, he (to return) home.
When his co-worker (to leave), he (to hurry) to the station to book a ticket.
e) Past Simple, Past Continuous. и Past Perfect.
1. When he (to enter) the room, his colleagues (to discuss) the plan which he (to make up) in the morning.
2. I (to sit) in an armchair and (to think) of my coming business trip when the door suddenly (to open) and an old friend of mine (to enter) the room.
3. He (to ask) me if I (can) give him your e-mail address.
4. She (to say) that he (to give) her the wrong address.
5. When I (to ring) up Mike, he still (to learn) the rule which he (to begin) learning at school.
6. I (to ask) him where he (to put) my letter.

C) 18. When he was painting the wall, he used scaffolding. 19. When I was sitting in the office, I suddenly saw my old friend. 20. When Nick was drafting a plan, his computer crashed. 9. I was signing the contract when my boss opened the door. D) 1. When they entered the hall, the conference had already begun. 2. The teacher understood that Helen hadn't done her homework. 3. He thought that he had lost the money. 4. Leo said that he had got (или gotten если американский английский) his education in Cape Town. 5. By the time we came to see him, he had returned home. 6. When his co-worker had left, he hurried to the station to book a ticket. e) Past Simple, Past Continuous. и Past Perfect. 1. When he entered the room, his colleagues were discussing the plan which he had made up in the morning. 2. I was sitting in an armchair and thinking of my coming business trip when the door had suddenly opened and an old friend of mine entered the room. 3. He asked me if I could give him your e-mail address. 4. She said that he had given her the wrong address. 5. When I rang up Mike, he was still learning the rule which he had begun learning at school. 6. I asked him where he had put my letter.