Learn Hardcore Finnish: Unohdin passini kotiin, mutta matkalaukkuni oli jo junassa. - I forgot my passport at home, but my suitcase was already on the train.

Verbs like unohtaa take a “total object” when the action is complete. You’ve wholly forgotten your passport, so the object appears in the nominative/accusative, not the partitive. In addition, -ni is the possessive suffix meaning “my,” so passini = “my passport.”

Why is kotiin in the illative case instead of saying kotona?

Illative (-in) marks movement or placement into a location. With verbs such as unohtaa and jättää, Finnish uses the illative to show where something ends up: unohtaa passin kotiin = “to forget one’s passport at home.” In contrast, kotona (inessive) would mean “while/when at home,” not “left at home.”

Why is matkalaukkuni in the nominative case (with -ni) instead of another case?

In the second clause matkalaukkuni is the grammatical subject of oli (“was”), so it stays in the nominative. The suffix -ni again marks possession: “my suitcase.”

What does junassa mean, and why is it in the inessive case (not adessive)?

Junassa is the inessive form of juna, meaning “inside the train.” Inessive -ssa/ssä indicates location within something. If you used the adessive (junalla), it could mean “by train” (as a means of transport) or literally “on the surface of the train,” which isn’t what you want here.

Could I say Jätin passini kotiin instead of Unohdin passini kotiin?

Yes, jättää (“to leave [behind]”) + illative also works: Jätin passini kotiin = “I left my passport at home.” The nuance is slightly different: unohtaa emphasizes that you forgot it, jättää is more neutral about the reason but still means you left it behind. Both take kotiin.

Why is there a comma before mutta, and can I omit it?

Standard Finnish punctuation separates two independent main clauses with a comma before conjunctions like mutta (“but”). It clarifies the contrast between clauses. In very informal writing you might drop it, but it’s recommended in most contexts.

Can I change the word order, for example start with Kotiin?

Finnish word order is relatively free for emphasis. You could say Kotiin unohdin passini, which stresses where you forgot it, but the neutral order is Unohdin passini kotiin (subject/verb/object/adverbial). Moving elements around changes the focus.