Learn Farsi: اگر این شاخص بهتر بود، نرخ مشارکت نود درصد می‌شد. - If this index were better, the participation rate would be ninety percent.

This is a Type 2 (unreal) conditional. Just like English uses "were" instead of "is" for imaginary situations, Farsi uses the simple past بود (was/were) in the "if" clause. For the result clause, Farsi adds the continuous prefix می to the simple past شد (became) to create می‌شد, which translates to "would become" or "would be".

How is the percentage نود درصد structured, and how does it affect the verb?

It translates exactly word-for-word as نود (ninety) and درصد (percent). In this sentence, the entire phrase نود درصد acts as the complement describing the subject. Because the logical subject is the singular noun نرخ (rate), the verb می‌شد takes the singular third-person form, regardless of the large number.

Why use a form of شدن (to become) instead of بودن (to be) for "would be"?

In Farsi conditional sentences, the result clause often uses the past continuous form of شدن (می‌شد) to express "would be" or "would happen". While English says "would be ninety percent", Farsi strongly prefers the dynamic nuance of "would become ninety percent" for hypothetical outcomes.

How do the words نرخ مشارکت link together to mean "participation rate"?

They are linked by the Ezafe, an unwritten "e" sound between the words (nerkh-e moshārekat). Farsi places the main noun (rate) first, followed by the noun acting as a modifier (participation), which literally translates to "rate of participation".