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.