Breakdown of Hän palauttaa kirjan viimeistään huomiseen mennessä.
kirja
book
hän
he / she
palauttaa
to return (an object, give back)
mennessä
by (a deadline)
Questions & Answers about Hän palauttaa kirjan viimeistään huomiseen mennessä.
Why is huomiseen in the illative case (ending in -een) here?
The postposition mennessä (by a deadline) requires the word before it to be in the illative case. Even though in English we say it happens by tomorrow, in Finnish you are grammatically saying it happens by the time we get into tomorrow.
Could I use asti or saakka instead of mennessä in this sentence?
No, because mennessä expresses a deadline for a completed action. Asti and saakka express continuous duration. If you used asti, it would sound like he is continuously in the process of returning the book until tomorrow!
Why is the object kirjan (ending in -n) and not the partitive kirjaa?
Returning a book is a completed action where the whole book is returned. Because of this, it takes the total object case (which looks exactly like the genitive with its -n ending). You would use the partitive if the action were ongoing or incomplete.
What is the word viimeistään and how does it relate to the deadline?
It is an adverb meaning at the latest, derived from the adjective viimeinen (last). It is extremely common to pair viimeistään with mennessä to emphasize that the deadline is an absolute limit.