Learn Finnish: Kunpa tarina olisi totuudenmukainen! - If only the story were truthful!

The particle kunpa means "if only" or "I wish," and it grammatically requires the verb to be in the conditional mood, which is recognized by the -isi- marker. Together, they express a strong, hypothetical wish for something that isn't currently true.

How is the adjective totuudenmukainen constructed?

It is a compound word made of totuuden (the genitive form of totuus, meaning "truth") and the suffix-like word -mukainen (meaning "in accordance with" or "conforming to"). Literally, it means "conforming to the truth." The -mukainen ending is very useful to learn, as it appears in many B2-level compound adjectives.

Could I use Toivon, että... (I hope that...) to express this same idea?

Yes, you could say Toivon, että tarina on totuudenmukainen. However, using kunpa plus the conditional mood creates a much stronger, more emotional exclamation, perfectly matching the English translation "If only...!" rather than a simple statement of hope.

Why is totuudenmukainen in the nominative case here rather than the partitive?

When using the verb olla (to be), the adjective describing the subject usually takes the nominative case if the subject is a singular, countable noun like tarina (a story). If the subject were an uncountable mass noun or an abstract concept, the adjective would take the partitive case.