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Stonehenge is perhaps the world's most famous prehistoric monument. It was erected in several stages.
«It was erected» – он был возведён (тогда был возведён) (Past Simple Passive)

ERECT

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The first monument was an early henge monument, built about 5,000 years ago.
«The first monument was an early henge monument» - Первым памятником был ранний памятник Хенджу. (числительные)

ONE

21

Later, in the Neolithic period about 2500 BC, there appeared the unique stone circle. Today, together with Avebury, Stonehenge forms the heart of a World Heritage Site, with a unique concentration of prehistoric monuments.
«Later, in the Neolithic period» –  позже в период неолита (Степени сравнения)

LATE

22

The earlier structures known in the immediate area are four or five pits, three of wich appear to have held large pine 'totem-pole like' posts erected in the Mesolithic period.
«The earlier structures known» - Ранее известные структуры (Степени сравнения)

EARLY

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It is not known how these posts relate to the monument of Stonehenge. At this time, when much of the rest of southern England was largely covered by woodland, the chalk downland in the area of Stonehenge may have been an unusually open landscape.
«It is not known how these posts» – Известно как эти колоны. (общее утверждение, вообще известно) (Present simple Passive)

NOT KNOW

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It is possible that this is why it became the site of an early Neolithic monument complex dating back to the period between 8500 and 7000 BC.
«Neolithic monument complex dating back to the period» - Комплекс памятников неолита, относящийся к тому периоду (относящийся) (Participle I)

DATE