Breakdown of Если бы не ваше упорство, проект бы рухнул.
ваш
your [formal/plural] (ваш, ваша, ваше)
проект
the project
бы
would (conditional particle) / concessive particle
если бы не
if it weren't for
Questions & Answers about Если бы не ваше упорство, проект бы рухнул.
Why is ваше упорство in the Nominative case? Doesn't the negative не usually require the Genitive when talking about the absence of something?
Normally, yes. But если бы не is a special fixed phrase meaning 'if it weren't for' or 'but for'. In this specific construction, the thing that prevented the bad outcome is always put in the Nominative case, not the Genitive.
Why do we need the second бы in проект бы рухнул?
The first бы belongs to the 'if' clause (Если бы не...), setting up the unreal condition. The second half of the sentence describes the hypothetical result, so it also needs to be a standard conditional clause. In Russian, that means pairing the past tense verb (рухнул) with its own бы.
Could I change the word order in the second half to проект рухнул бы?
What makes рухнул the right verb aspect to use here instead of an imperfective equivalent?
Рухнул is perfective, focusing on the completed result: the total, one-time collapse of the project. If you used an imperfective verb, it would mean the project 'would be in the process of collapsing,' which doesn't fit the sudden, final meaning intended here.