Learn Finnish: Hän jäi orvoksi, joten hän asui isoäidin luona. - He became an orphan, so he lived with his grandmother.

The verb jäädä (usually "to stay" or "to remain") requires the translative case when it describes a resulting state or condition. While the English translation is "became an orphan," the Finnish expression literally means being left in the state of an orphan.

Could I use tuli (became) instead of jäi? For example, hänestä tuli orpo?

Yes, hänestä tuli orpo is grammatically correct. However, jäädä combined with the translative case (jäi orvoksi) is the most natural idiom for this situation. It carries the emotional nuance of being left behind in a certain condition, which fits the context of losing one's parents better than the neutral verb tulla.

What causes the "p" in orpo to change to "v" in orvoksi?

This is a standard consonant gradation (p changes to v). Adding the translative suffix -ksi closes the final syllable of the word. Because the syllable becomes closed, the strong grade p softens to the weak grade v.

Why use the postposition luona instead of just the adessive case, like asui isoäidillä?