Breakdown of Tapaus oli ratkaisematon, koska vihjettä ei ollut huomattu.
olla
to be
koska
because
ei
no / not (en, et, ei, emme, ette, eivät)
huomata
to notice
Questions & Answers about Tapaus oli ratkaisematon, koska vihjettä ei ollut huomattu.
Why is the auxiliary verb ollut and not oltu in ei ollut huomattu?
This is a very common trap! In standard Finnish, the negative pluperfect passive uses the active participle ollut, not the passive oltu. The structure is ei + ollut + the passive past participle (huomattu). While you will often hear ei oltu huomattu in colloquial spoken Finnish, standard written Finnish requires ei ollut huomattu.
If passive sentences usually take a nominative object, why is vihjettä in the partitive case here?
You are right that a complete object in an affirmative passive sentence takes the nominative case (for example, Vihje oli huomattu). However, the golden rule that negative sentences require a partitive object overrides the passive object rules. Because the action didn't happen (ei ollut huomattu), the word vihje (clue) must take the partitive form vihjettä.
How is the adjective ratkaisematon (unsolved) formed?
It is derived from the verb ratkaista (to solve). By taking the strong verb stem ratkaise- and adding the negative suffix -maton (or -mätön), it creates an adjective meaning "without doing" the action—in this case, "unsolved" or "unsolvable".
Could I just use the imperfect tense and say koska vihjettä ei huomattu?