Learn Finnish: Orjat lienee jätetty tänne. - The slaves were probably left here.

It uses the auxiliary verb lienee (the potential form of the verb to be) followed by the passive past participle jätetty (from the verb jättää). Together, they express that something was probably done to the object in the past.

Since orjat is plural, why isn't the auxiliary verb in the plural form lienevät?

The passive voice in Finnish is impersonal and never agrees with the noun. Even though orjat translates as the subject in the English sentence, grammatically it is the object of the passive verb, so the auxiliary verb must remain in the default singular lienee.

Why is orjat in the nominative plural instead of the partitive?

In an affirmative passive sentence, a total object (indicating a completed action affecting the whole group) takes the nominative case when it is plural. If the action were ongoing or only affected an unspecified number of slaves, we would use the partitive plural orjia.

Why does the sentence use tänne (to here) instead of täällä (at here)?

The verb jättää (to leave) is a directional verb in Finnish. It requires a destination case indicating where something is moving to, rather than a static location case. You leave things into or onto a place, which is why the directional tänne is used.