Learn Finnish: Todistaja sanoi, että autoa ei ollut nähty täällä. - The witness said that the car had not been seen here.

It is built using three parts: the negative word ei, the auxiliary verb ollut (the past participle of olla), and the main verb nähty (the passive past participle of nähdä). The trickiest part for learners is that the auxiliary verb uses the active participle ollut rather than the passive oltu, even though the main verb is passive.

Why is autoa (the car) in the partitive case here?

Because it is the object of a negative sentence. In Finnish, if the action is negative, the object almost always takes the partitive case to show that the action did not happen to it. If the sentence were positive ("The car had been seen"), it would be in the nominative case as the object of a passive verb: Auto oli nähty.

Why does the sentence use the pluperfect tense (ei ollut nähty) instead of the simple past passive (ei nähty)?

The pluperfect ("had not been") is used to show that a past action happened before another action in the past. The witness "said" (sanoi) something in the past, and they were reporting on a timeframe that occurred even earlier.

Why use täällä for "here" instead of tässä?