Learn Finnish: Jos raketti olisi räjähtänyt, kiertorataa ei olisi saavutettu. - If the rocket had exploded, the orbit would not have been reached.

It consists of three parts: the negative word ei, the conditional helper verb olisi, and the passive past participle saavutettu. Notice that the helper verb is always olisi in the passive, regardless of who or what the sentence is about.

Why is the object "kiertorataa" in the partitive case?

Because the action in that clause is negative (ei olisi saavutettu). In Finnish, the object of a negative sentence is always in the partitive case. If the sentence were positive ("the orbit would have been reached"), the object would be in the nominative: kiertorata.

Both clauses use "olisi", but why does one verb end in "-nyt" (räjähtänyt) and the other in "-ttu" (saavutettu)?

They use different past participles because the first clause is active and the second is passive. Räjähtänyt is the active past participle, showing the rocket itself exploded. Saavutettu is the passive past participle, showing the orbit was reached by an unspecified actor. Active past participles usually end in -nut or -nyt, while passive ones end in -ttu, -tty, -tu, or -ty.

What is the base form of the verb used in "saavutettu"?

The base verb is saavuttaa, meaning "to reach" or "to achieve". Since it is a transitive verb (a verb that takes an object), it can be put into the passive voice to describe an action done to something.