This tense is formed by had followed by the Past Participle:
She had written.
Uses:
To refer to an action in the past that continues up to, or relates to, a single point in the past:
By 1428 they had established a city state.
To contrast two events, one of which happened before the other:
I went round after I had finished.
To show a causal link between two events in the past:
She remembered because she had seen her drive off.
In narrative to give background information:
It had been a bad year for Cliff.
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