Palautan kirjan kirjastoon huomenna.

Finnish doesn’t have a separate future tense. The simple present covers both present and future, and time words like huomenna (tomorrow) give the future meaning. You can make the futurity more explicit with:

• Aion palauttaa kirjan huomenna. (I intend to return the book tomorrow.)
• Palautan kirjan huomenna. is already perfectly natural for “I will return...”.

Finnish objects alternate mainly between “total” and “partitive” forms.

• Kirjan is a total object (formally genitive singular used as accusative). It’s used when the action is bounded/completed: returning the whole book.
• Kirjaa (partitive) appears with negation, ongoing or unbounded actions, or with “some (amount of) books” meanings.
  • Negation: En palauta kirjaa huomenna.
  • Indefinite plural/habitual: Palautan kirjoja usein. (I return books often.)

Finnish uses local cases instead of prepositions:

• kirjastoon = illative “into the library” (movement into).
• kirjastossa = inessive “in the library” (location, no movement).
• kirjastolle = allative “onto/to the library (as a recipient or to the vicinity)”. Here, you’re moving the book into the library, so kirjastoon is correct.

Illative (into) is often made by lengthening the final vowel and adding -n.

• kirjasto → kirjastoon (the final o becomes oo
  • n) Other examples: talo → taloon, koulu → kouluun.

Finnish verbs have personal endings, so the subject is encoded in the verb:

• palauta-n = “I return.” Adding minä is optional and used for emphasis or contrast: Minä palautan... (I, as opposed to someone else, will return...).

The ending -n marks 1st person singular. Present tense of palauttaa:

• minä palautan
• sinä palautat
• hän palauttaa
• me palautamme
• te palautatte
• he palauttavat Note how -tt- appears in the 3rd person forms (palauttaa, palauttavat) but surfaces as -t- in others (a regular alternation).

Yes. Finnish word order is flexible; elements are moved for emphasis/topicalization.

• Neutral: Palautan kirjan kirjastoon huomenna.
• Emphasize time: Huomenna palautan kirjan kirjastoon.
• Emphasize place: Kirjastoon palautan kirjan huomenna. The case endings keep roles clear, so movement doesn’t cause ambiguity.

Use plural forms:

• Total object (all of them): Palautan kirjat kirjastoon.
• Partitive plural (some/unspecified quantity or habitual): Palautan kirjoja kirjastoon.

Common options:

• Intention/plan: Aion palauttaa kirjan huomenna.
• “Thinking of” (plan, tentative): Ajattelin palauttaa kirjan huomenna. Simple present with huomenna is already the default way to talk about the future.