Learn Finnish: Pidätkö sitä mahdollisena? - Do you consider it possible?

When the verb pitää is used to mean 'to consider' or 'to regard as', it requires its object to be in the partitive case. Here, the object is se (it), which becomes sitä.

What is the -na ending on mahdollisena doing?

That is the essive case ending. In Finnish, when you use pitää to say you consider something to be a certain way, the adjective or noun describing it must be in the essive case. So the basic word mahdollinen (possible) takes the -na ending.

Pitää can also mean 'to like'. How can I tell it means 'to consider' here?

The case endings give it away! If it meant 'to like', the object would be in the elative case, ending in -sta or -stä, like Pidätkö siitä? (Do you like it?). Because we have a partitive object (sitä) and an essive word (mahdollisena), you know the verb means 'to consider'.

The current lesson also mentions luulla. How would this sentence look if we used luulla (to think/suppose) instead?

If you used luulla, the adjective would change from the essive case (-na) to the translative case (-ksi). The sentence would become Luuletko sitä mahdolliseksi? This leans slightly more towards 'Do you presume it to be possible?' rather than expressing a steady, considered opinion.