Learn Finnish: Lohkare lienee vieritetty puiselle alustalle. - The boulder was likely rolled onto a wooden platform.

It combines the active potential form of the helper verb olla (lienee, meaning "is probably") with the passive past participle of the main verb (vieritetty, meaning "rolled"). Together, they mean "was likely rolled" or "has probably been rolled". In the passive potential perfect, only the main verb takes the passive marker.

Since the boulder is the object being rolled, why is lohkare in the nominative case (the basic dictionary form)?

In Finnish passive sentences, a total object—one that undergoes a complete action—looks exactly like the nominative case rather than taking a genitive-like accusative ending. Since the rolling of the boulder is a completed action, lohkare stays in its basic form.

Why is the verb vierittää used here instead of vieriä? Both translate to English as "to roll".

Finnish is strict about separating transitive verbs (doing an action to an object) from intransitive verbs (something happening on its own). Vierittää is transitive, implying someone or a machine rolled the boulder. If the boulder had just rolled by itself, you would use the active form of the intransitive verb vieriä: lohkare lienee vierinyt (the boulder probably rolled).

Why are the words puiselle alustalle in the allative case (-lle)?