by the way, the sentence listed upwards is already correct.

little mice wrote:OG13
89.Salt deposits and moisture threaten to destroy the
Mohenjo-Daro excavation in Pakistan, the site of an
ancient civilization that flourished at the same time
as the civilizations in the Nile Delta and the river
valleys of the Tigris and Euphrates.

The explanation in OG says flourished uses the simple past tense.

My confusion: since "flourished" is a past tense, then threaten appearing in the same sentence should be "threatened", too.

Correct me!

Salt deposits and moisture still continues to threaten the Mohenjo-Daro excavation in Pakistan. Therefore it is in simple present tense.

This threating is due to some civilization that flourished sometime in the past, therefore it is in simple past tense.

Hope that helps !!

little mice wrote:OG13
89.Salt deposits and moisture threaten to destroy the
Mohenjo-Daro excavation in Pakistan, the site of an
ancient civilization that flourished at the same time
as the civilizations in the Nile Delta and the river
valleys of the Tigris and Euphrates.

The explanation in OG says flourished uses the simple past tense.

My confusion: since "flourished" is a past tense, then threaten appearing in the same sentence should be "threatened", too.

Correct me!

You need to look holistically at what the sentence is trying to convey, not just at the grammar.
Ancient civilizations at Mohenjo-Daro existed in the past - therefore, 'flourished' must use the past tense.
What is the issue at hand?
This excavation is currently being threatened by salt deposits and moisture - this is a present-day problem. Therefore, 'threaten' should be in the simple present tense.
I hope the difference is clear.