Learn Hardcore Finnish: Kirjani on pöydällä, mutta ystäväni valitsi toisen. - My book is on the table, but my friend chose another.

-ni is the possessive suffix meaning “my.” In Finnish you normally attach it to the last case form of the noun: kirja (nominative) → kirjan (genitive) → kirjani (my book). Yes, you could alternatively say minun kirjani using the separate pronoun minun, but the suffix version is more idiomatic and concise.

What case is pöydällä, and why not pöydälle or just pöytä?

pöydällä is the adessive case, marked by -lla/llä, which expresses location “on” something. So kirjani on pöydällä literally means “my book is on [the] table.”
By contrast, pöydälle (allative case) means “onto the table” (movement), and plain pöytä (nominative) would lack any “on” or “onto” meaning.

Why is there no article like “the” before pöytä?

Finnish doesn’t have definite or indefinite articles like the or a. Definiteness is inferred from context or word order. Here, pöydällä simply means “on the table” in English, but in Finnish you don’t need a separate article.

Why is the second clause ystäväni valitsi toisen, and what case is toisen in?

ystäväni means “my friend” (with the same possessive suffix -ni as before). valitsi is the 3rd person singular imperfect (past) of valita (“to choose”). toisen is a pronoun meaning “another one” or “the other one.” Grammatically it’s in the nominative (or accusative) form, since Finnish singular 3rd-person accusative equals the nominative for inanimate objects. You don’t need to repeat kirja, because toisen stands in for “another [book].”

Could you say ystäväni valitsi toisen kirjan instead?

Yes, you could say ystäväni valitsi toisen kirjan, which more explicitly means “my friend chose the other book.” In that case, toisen is the adjectival form of toinen agreeing with kirjan in the genitive (since the object of valita can take either genitive or accusative in Finnish). But omitting kirjan is common when context makes the noun clear.

Why is valitsi used here instead of the present tense valitsee?

Because the action happened in the past (your friend already chose a different book), you use the imperfect tense valitsi. valitsee would mean “chooses” (present/future), which would change the meaning to a habitual or ongoing choice.

Is the word order fixed, or could I say Pöydällä on kirjani?

Finnish word order is relatively flexible, thanks to its case system. You can say Pöydällä on kirjani (“On the table is my book”) to emphasize location or style, but the meaning stays the same. The given sentence Kirjani on pöydällä follows the neutral Subject–Verb–Adverbial order.