Learn Finnish: Jos lentoa ei olisi keskeytetty, satelliitti olisi räjähtänyt. - If the flight had not been aborted, the satellite would have exploded.

It has three parts: the negative word ei, the conditional form of the verb "to be" (olisi), and the passive past participle of the main verb (keskeytetty). Notice that in the passive, the helper verb olisi always stays in the third-person singular form, regardless of what is being acted upon.

Why is the word for flight (lentoa) in the partitive case here?

Because it is the object of a negative sentence. In a positive passive sentence, the object would typically be in the nominative case (for example, Lento olisi keskeytetty). However, whenever a passive sentence is made negative, its object must switch to the partitive case.

The English translation says "had not been aborted", so why does the Finnish use the conditional olisi instead of the past perfect oli?

When talking about hypothetical past events, English uses the past perfect in the "if" clause ("If it had not...") and the conditional in the main clause ("...it would have"). Finnish is more consistent here: it uses the conditional perfect in both halves of the sentence. Literally, it says "If the flight would not have been aborted, the satellite would have exploded."

Why does the first clause use a passive participle (keskeytetty) while the second uses an active one (räjähtänyt)?