Kirjasto lähettää muistutusviestin kaksi päivää ennen eräpäivää.

Yes. Kirjasto is the subject and it’s in the nominative (the basic dictionary form). Finnish typically doesn’t mark the subject with a special ending in simple sentences like this; the “subject-ness” is shown by word order and verb agreement.

The doer is Kirjasto (the library), and the verb lähettää is in 3rd person singular, which matches kirjasto:

• lähettää = (it) sends

So the subject + verb agreement supports that Kirjasto is doing the sending.

Finnish present tense often covers:

• habitual actions (what usually happens), and
• scheduled/expected future actions (especially in contexts like services and routines)

So Kirjasto lähettää... can naturally mean the library sends (as a rule / typically), even if the actual sending happens on a later date.

Because it’s a “total object” (a complete, bounded action: sending one whole reminder message). In the indicative present like this, a singular total object usually appears as:

• genitive-looking -n: muistutusviestin

Compare:

• lähettää muistutusviestin = sends the reminder message (a complete item)
• lähettää muistutusviestiä = sends reminder message / is sending some reminder message (partial/ongoing/indefinite)

Functionally it’s the object case (often called “accusative object”), but in Finnish the singular total object is typically marked like the genitive (-n) in most normal affirmative sentences:

• muistutusviestin looks genitive, but it’s serving the total-object role here.

Finnish “accusative” is mostly visible with personal pronouns (e.g., minut, sinut) and in some specific constructions; otherwise the object marking overlaps with genitive/nominative/partitive patterns.

It’s a compound noun:

• muistutus = reminder
• viesti = message
• muistutusviesti = reminder message

Then it’s inflected as one word:

• nominative: muistutusviesti
• total object here: muistutusviestin

After numbers greater than one, Finnish typically uses the partitive singular for the counted noun:

• kaksi päivää = two days
• kolme kirjaa = three books

So päivää is the partitive singular of päivä.

Because ennen is a postposition that governs the partitive case:

• ennen eräpäivää = before the due date

Many Finnish postpositions require a specific case, and ennen + partitive is a key one to memorize.

Yes, Finnish word order is flexible, but the given order is neutral and typical:

• Kirjasto lähettää muistutusviestin kaksi päivää ennen eräpäivää.

You could move the time phrase for emphasis:

• Kaksi päivää ennen eräpäivää kirjasto lähettää muistutusviestin. (emphasizes the timing)

The cases keep the meaning clear even if you reshuffle elements.