When you're telling a story about the past and need to talk about something that happened even earlier — before another past event — you need the past perfect, not the simple past. Compare: 'When I arrived, the train left.' (sounds like it left right when you arrived) vs 'When I arrived, the train had left.' (it left before you got there — that's why you missed it).

The form is: had + past participle. It's the 'past of the past' — use it to make the order of two past events completely clear. 🔊