Use the past perfect tense to express states oractions that began and came to an end before a specific point in the past. To form the past perfect, use the past tense form of theauxiliary have and the
By 150 B.C, the Greeks had accuratelydetermined the shape and dimensions of the Earth and thedistance of the Moon, but had not managedto probe very far beyond that.
--Isaac Asimov, The Universe
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