Question 1

Why is the -kin ending attached to tuo instead of kirja?

Accepted Answer

The clitic -kin ("also" or "too") usually attaches to the exact word you want to emphasize. By putting it on tuo (that), the sentence highlights which one you mean: "That book is interesting too" (because we already know this other one is). While you could technically say tuo kirjakin to emphasize the word "book", it is very common in Finnish to attach the clitic to the first word of a noun phrase.

Question 2

Could I attach the ending to the adjective instead, like Tuo kirja on kiinnostavakin?

Accepted Answer

You could, but it would change the meaning. The -kin ending acts like a spotlight. If you put it on the adjective, it means "That book is interesting too" (for example: we already know it has a beautiful cover, but it turns out it is also interesting to read). Attached to tuo, it means the book is adding to a list of other interesting books.

Question 3

Why is kiinnostava in the basic nominative case here instead of the partitive case (kiinnostavaa)?

Accepted Answer

Because kirja (book) is a singular, countable object. When describing a whole, countable noun with the verb olla (to be), the adjective must be in the basic nominative case. You would only use the partitive case if the subject was an uncountable mass word, like water or music, or an abstract idea.

Question 4

Is tuokin related to the pronoun jokin (something)?

Accepted Answer

They look similar because they both use -kin, but they work differently. Jokin is a fixed pronoun where the ending is permanently glued on. In tuokin, the -kin is just our temporary "also/too" ending attached to the normal demonstrative word tuo (that).

Tuokin kirja on kiinnostava.

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