The sentence from my grammar book:
When he had read the letter he burned it.
Why not just "when he read "? These are consequntive actions, aren't they?
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With both actions in the simple past, the two actions are presented one after the other but the listener is free to understand them as sequential or as simultaneous. Whether they are sequential or simultaneous is a semantic choice, not a grammatical one.
He picked up the cup and took a sip. sequential
He stood on one foot and recited the alphabet backwards. simultaneous?
Our semantic choice is governed by our sense of whether the verbs are goal-oriented. Does the action have an end?
A speaker has a third choice, actions that are not simply sequential, and not simply simultaneous:
He took her hand in his and asked her to marry him.
When he had taken her hand in his, he asked her to marry him.
With the simple past, we are free to understand the two actions as sequential or as simultaneous. He took her hand in his and (then?) popped the question.
With the perfect, those two actions are sequential and interrelated, the first action, his taking her hand in his, was achieved before the second action occurred.
In order to understand the perfect, you must understand what it means to present an action as one that is achieved or complete. The act of his taking her hand in his was fully realized. It was not beginning, nor was it in progress. It had been done.
The perfect establishes a point in time in respect to which other actions are situated in time.
Past Perfect here deploys the state of a completed action that took place before another action in the past. This means that "He first read the letter and then he burned it"
If we say "When he read the paper he burned it" it may mean that during reading he burned it but may also mean that when he finished reading it he burned it so this leaves us with ambiguity. In order to filter it out and tell us precisely what came after what the author used the Past Perfect tense.