Learn Finnish: Netti katkesi, joten minun täytyy käyttää puhelinta. - The internet disconnected, so I have to use the phone.

Both can translate to "so" or "therefore," but they connect ideas differently. joten is a relative conjunction that directly links the consequence to the previous clause in the same sentence, following a comma. siksi is an adverb meaning "for that reason," and it usually starts a brand new sentence or pairs with koska (because).

Why do we use katkesi instead of katkaisi for the internet disconnecting?

katkesi is the past tense of katketa, which means something snaps, breaks, or gets cut off on its own. Because the internet dropped out by itself, we use this verb. If you used katkaisi (from katkaista), it would mean someone actively and intentionally cut the connection.

Why is the subject minun instead of minä?

This is a refresher on "necessity sentences." The verb täytyy (to have to / must) requires the person who has to do something to be in the genitive case. Therefore, the basic pronoun minä changes to minun.

In a necessity sentence with täytyy, doesn't the object usually take the basic nominative form? Why is puhelinta in the partitive case?