The phrase "had lost" is in the past perfect tense, which is used to indicate an action that was completed before another past action. In this case, the event of losing something happened first, before the realization occurred. Therefore, "had lost" is the correct choice to show that this event happened before the moment of realization.
Step 2: Understanding the Simple Past Tense
The word "realised" is in the simple past tense, which describes an event or action that happened at a specific moment in the past. After the person had lost something, they then realized it. The action of realizing comes after the loss, so "realised" is the correct verb for this second part of the sentence.
Conclusion:
The correct use of "had lost" in the past perfect tense shows the event that happened first.
"Realised" in the simple past tense correctly indicates the moment the person understood what had happened afterward.