1. I (to sit) in an armchair and (to think) of rny coming trip across the North Sea when the door suddenly...

Sure, here are the sentences completed using appropriate verb forms in the past simple or past continuous tenses, or other tenses where applicable:

1. I was sitting in an armchair and thinking of my coming trip across the North Sea when the door suddenly opened and an old friend of mine whom I had not seen for a very long time entered the room.
2. She came to see us just at the time when we were having dinner. It was the first time I had seen her.
3. I saw him just as he was leaving the hotel.
4. I had not seen him before we met at the concert.
5. He had left the house before I had time to ask him anything.
6. He told me he had learned it from the newspaper.
7. He entered the room, took something from the desk, and went out.
8. There were two men in the room. One of them was writing something while the other was reading a newspaper.
9. He had not told me that he had received a telegram from her.
10. I asked him if he knew where she lived. I said I did not know her address.
11. He asked me if I could give him your address.
12. She said that he had given her the wrong address.
13. I asked him where he had put my letter.
14. He told us that they had spent all the money.
15. After spending several days in Paris, he felt lonely and wanted to return home.
16. I thought he had already gone home.
17. I found the old man in the garden. He was talking to some children who were standing around listening to him.
18. He spoke a language we had never heard before.

These sentences use a mix of past simple for actions completed at a specific time in the past, past continuous for actions that were ongoing, and past perfect for actions that had been completed before another action or specific time in the past.

1. I was sitting in an armchair and thinking of my coming trip across the North Sea when the door suddenly opened and an old friend of mine whom I had not seen for a very long time entered the room.
2. She came to see us just at the time when we were having dinner. It was the first time I saw her.
3. I saw him just as he was leaving the hotel.
4. I had not seen him before we met at the concert.
5. He left the house before I had time to ask him anything.
6. He told me he had learned it from the newspaper.
7. He entered the room, took something from the desk and went out.
8. There were two men in the room. One of them was writing something while the other was reading a newspaper.
9. He did not tell me that he had received a telegram from her.
10. I asked him if he knew where she lived. I said I did not know her address.
11. He asked me if I could give him your address.
12. She said that he had given her the wrong address.
13. I asked him where he had put my letter.
14. He told us that they had spent all the money.
15. After spending several days in Paris he felt lonely and wanted to return home.
16. I thought he had already gone home.
17. I found the old man in the garden. He was talking to some children who were standing around listening to him.
18. He was speaking a language we had never heard before.

1. I was sitting in an armchair and thinking of my coming trip across the North Sea when the door suddenly opened and an old friend of mine whom I had not seen for a very long time entered the room.
2. She came to see us just at the time when we were having dinner. It was the first time I had seen her.
3. I saw him just as he was leaving the hotel.
4. I had not seen him before we met at the concert.
5. He left the house before I had time to ask him anything.
6. He told me he had learned it from the newspaper.
7. He entered the room, took something from the desk and went out.
8. There were two men in the room. One of them was writing something while the other was reading a newspaper.
9. He did not tell me that he had received a telegram from her.
10. I asked him if he knew where she lived. I said I did not know her address.
11. He asked me if I could give him your address.
12. She said that he had given her the wrong address.
13. I asked him where he had put my letter.
14. He told us that they had spent all the money.
15. After spending several days in Paris, he felt lonely and wanted to return home.
16. I thought he had already gone home.
17. I found the old man in the garden. He was talking to some children who were standing around listening to him.
18. He was speaking a language we had never heard before.