Learn Finnish: Pystytkö pelastamaan hänet, jos katto romahtaa? - Are you able to save him if the roof collapses?

The verb pystyä (to be able to / to have the ability) always requires the following verb to be in the third infinitive illative form, ending in -maan or -mään. If you had used the more general auxiliary verb voida (can), it would take the basic first infinitive: Voitko pelastaa...

Why is the object hänet instead of the partitive häntä?

The verb pelastaa (to save) takes a total object because the action is seen as a completed, successful event rather than an ongoing process. For personal pronouns like hän, the total object takes the special accusative ending -t, becoming hänet.

How does pystytkö differ from voitko in this context?

Both can translate to "can you" in English, but pystyä specifically asks about having the physical or mental capability, strength, or resources to do something challenging. Voida is broader and often asks about possibility, permission, or general circumstances.

Could you use a different verb for the roof falling, like pudota or kaatua?

Not naturally. Romahtaa specifically means "to collapse" or "to cave in", which is exactly what a roof or building does. Pudota means to fall from a height (like a book from a table), and kaatua means to tip over or fall down (like a tree or a person).