When did you realise it? --- At 11 o'clock.

When did you leave the water bottle at home? --- Earlier, at 8 o'clock.

Use Past Perfect tense when you talk about the past and something happened even earlier:

• It was raining, but I hadn't brought my umbrella.
• My cat stole my sandwich I had put on the table last night.
• I carried the bag my mom had bought in the city.

You can also recognise from these words, when to use Past perfect:

• When I called him on my phone he had already eaten his dinner.
• I had deleted his file before he logged in.
• By the time the storm came, the family had finished their lunch.