Past perfect simple or past simple?

Past perfect simple: I had seen Past simple: I saw

The past perfect simple is used to sequence events in the past to show which event happened first.

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The music *started* when the curtains *opened*.	Past simple + past simple: the music started at the same time as the curtains opened.
The music *had* (already) *started* when the curtains *opened*.	Past perfect simple + past simple: the music started and then the curtains opened.

The past simple often suggests a stronger connection between the time of the two events.

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When she *came* through the door, everyone *shouted*, ‘Surprise! Happy Birthday!’.	Past simple + past simple: this sentence shows that as she came through the door, everyone immediately shouted ‘Surprise!’.
When she *had read* all the greetings cards, she *made* a short thank-you speech.	Past perfect simple + past simple: this sentence emphasises that she had finished reading the cards before she made her speech.