We use the past perfect simple to talk about what happened before a point in the past. It looks back from a point in the past to further in the past.
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I hadn't known the bad news when I spoke to him.
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I checked with the supplier and they still hadn't received the contract.
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She had already told him before I got a chance to give him my version.
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The company has started the year well but was badly hit by the postal strike.
The past perfect simple is often used when we report what people had said/thought/believed.
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He told me they had already paid the bill.
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He said he believed that John had moved to Italy.
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I thought we had already decided on a name for this product.
We use the past perfect continuous to look back at a situation in progress.
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It looked like a good time to invest. Inflation had been falling for several months.
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Before I changed jobs, I had been working on a plan to reduce production costs.
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We had been thinking about doing that but then decided against it.