Merkitsetkö eräpäivän kalenteriin, jotta et unohda palauttaa kirjaa?

-kö/-ko is the Finnish question clitic that turns a statement into a yes/no question.

• Merkitset = You mark / you will mark
• Merkitsetkö? = Do you mark / Will you mark?
  Which form you use depends on vowel harmony: after back vowels (a o u) you typically get -ko, otherwise -kö.

Finnish doesn’t have a separate future tense. The present tense often covers both present and near-future depending on context:

• Merkitset can mean you mark or you will mark.

Kalenteriin is the illative case, meaning into / in(to) the calendar (i.e., “enter it into the calendar”).

• kalenteri = calendar
• kalenteriin = into the calendar
  Illative often answers “where to?” rather than “where?”

Yes, but it changes the nuance:

• kalenteriin (illative) = put/enter it into the calendar (destination)
• kalenterissa (inessive) = it’s in the calendar (location) / mark it while in the calendar
  For the idea of adding an entry, kalenteriin is the natural choice.

Jotta means so that / in order that and introduces a purpose clause. Finnish typically uses a comma before subordinate clauses like this:

• ..., jotta et unohda ... = ..., so that you don’t forget ...

Finnish uses a separate negative verb that is conjugated for person/number, and the main verb takes a special form:

• et = you don’t (2nd person singular)
• unohda = the connegative form of unohtaa (used after the negative verb)
  So et unohda literally works like you-don’t forget.

After verbs like muistaa/unohtaa (to remember/forget), Finnish commonly uses the A-infinitive (-a/-ä) to express “to do something”:

• unohtaa palauttaa = forget to return

Because unohtaa typically takes a partitive object when it means “forget (something)” in the sense of failing to do/remember it. So:

• unohtaa kirjaa/palauttaa kirjaa is natural in this “forget to return the book” meaning.
  Using kirjan would push toward a more “complete/definite object” reading and often sounds less natural in this specific structure.

The structure is:

• Verb + -kö (question) + object + place + jotta-clause
  Finnish word order is flexible for emphasis, but clitics like -kö must attach to the word being questioned. For example, you could front something for emphasis, but then -kö would attach to that: Kalenteriin-ko merkitset eräpäivän...? (more contrastive: Into the calendar, do you mark it...?).