Use the past perfect tense to express states or actions 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 the auxiliary have and the past participle of the following verb.
By 150 B.C, the Greeks had accurately
determined the shape and dimensions of the Earth and the
distance of the Moon, but had not managed
to probe very far beyond that.
--Isaac Asimov, The Universe