Use "had" or "will have": 1. when we came to the station the train……… left. 2. her i ……..waited for her till 5 o'clock and then left. 3. when ii walked in sam…… left the room. 4. we…… finished all preparations by the time the guests come. 5. he ………had a terrible day and looked very tired. 6. we
hope they landed safely in paris i by that time. 7. forgotten all about him by the time he returns. 8. the arrived before we were ready, 9. at last i learnt …….happened to him. 10. ….you finished your paper at last? 11. by the end of last month they…… not completed the experiment yet. 12.
left the house before you return.

1. When we came to the station, the train had already left.
Explanation: The action of the train leaving the station happened before we arrived at the station. Therefore, we use the past perfect tense "had left" to indicate that the train departed before our arrival.

2. I will have waited for her till 5 o'clock and then left.
Explanation: The action of waiting for her until 5 o'clock and then leaving will happen in the future. Therefore, we use the future perfect tense "will have waited" to express this.

3. When I walked in, Sam had already left the room.
Explanation: The action of Sam leaving the room occurred before I walked in. Therefore, we use the past perfect tense "had left" to indicate that Sam departed before my entrance.

4. We will have finished all preparations by the time the guests come.
Explanation: The action of finishing all preparations will happen in the future before the guests arrive. Therefore, we use the future perfect tense "will have finished" to express this.

5. He had a terrible day and looked very tired.
Explanation: The action of having a terrible day and looking tired both occurred in the past. Therefore, we use the past simple tense "had" to describe these situations.

6. We hope they will have landed safely in Paris by that time.
Explanation: The action of landing safely in Paris is expected to happen in the future before a certain point in time. Therefore, we use the future perfect tense "will have landed" to express this.

7. We will have forgotten all about him by the time he returns.
Explanation: The action of forgetting about him will happen in the future before he returns. Therefore, we use the future perfect tense "will have forgotten" to indicate this.

8. They arrived before we were ready.
Explanation: The action of their arrival happened in the past before our readiness. Therefore, we use the past simple tense "arrived" to describe this event.

9. At last, I learnt what had happened to him.
Explanation: The action of learning what had happened to him occurred in the past. Therefore, we use the past perfect tense "had happened" to express this.

10. Have you finished your paper at last?
Explanation: The action of finishing the paper is happening in the present, so we use the present perfect tense "have finished" to inquire about it.

11. By the end of last month, they had not yet completed the experiment.
Explanation: The action of not completing the experiment happened in the past by the end of last month. Therefore, we use the past perfect tense "had not completed" to indicate this.

12. They left the house before you returned.
Explanation: The action of leaving the house happened in the past before your return. Therefore, we use the past simple tense "left" to describe this event.