Learn Hardcore Finnish: Määräaika näkyy kalenterissa punaisella, jotta en unohda sitä. - The deadline is shown in red in the calendar so that I do not forget it.

Together, määräaika means a fixed time limit, usually translated as deadline or due date. It implies there is a specific last possible time for something.

Aika on its own is more general: time, a moment, an occasion.
So:

määräaika = deadline
aika = time (in general), a moment, a period

Why is it näkyy and not something like on or näyttää?

Näkyä is an intransitive verb meaning “to be visible / to be seen / to appear” (from the viewer’s point of view).

Määräaika näkyy kalenterissa = The deadline is visible / appears in the calendar.

Compare:

olla: määräaika on kalenterissa – the deadline is in the calendar (just stating location)
näyttää: kalenteri näyttää määräajan – the calendar shows the deadline (transitive: the calendar “shows” something)

Using näkyy emphasizes that the deadline can be seen there, rather than just that it exists there.

Why is it kalenterissa and not kalenterissa vs kalenteriin / kalenterista / kalenterilla, etc.? What does this case mean?

Kalenterissa is the inessive case (ending -ssa / -ssä) and usually means “in, inside”.

The main local cases:

kalenterissa = in the calendar (inside it as a place/container for entries)
kalenteriin = into the calendar (movement towards: I enter it into the calendar)
kalenterista = from the calendar (movement out of: I remove it from the calendar)
kalenterilla = on/at the calendar (adessive, usually for surfaces or instruments; here it would sound odd)

Here, the deadline resides as an entry in the calendar, so kalenterissa fits.

Why is punaisella in the adessive (with -lla) and not something like punaisena or punaisessa?

Color used like this – “in red”, “in blue”, etc. – is commonly expressed with the adessive in Finnish:

punaisella = in red
sinisellä = in blue
vihreällä = in green

The idea is close to “with red (color)” or “by means of red”, so it uses the same case often used for instruments or means (adessive).

Alternatives and nuance:

punaisella – standard for written/displayed in red.
punaisena (essive) – literally as red; more about being in a red state, not usually used for “written in red” in this context.
punaisessa (inessive) – in the red (thing), odd here.

So for something shown / written / marked in a color, Finnish normally uses the adessive: punaisella.

Could I say Määräaika näkyy punaisena kalenterissa instead?

You could say it and be understood, but:

Määräaika näkyy kalenterissa punaisella is the natural, idiomatic way to say “The deadline appears in the calendar in red.”
punaisena would shift the focus more toward the state of being red rather than “highlighted/written in red”, and it’s not the usual way to describe color as a display/writing color.

Native speakers overwhelmingly prefer punaisella in this context.

What does jotta mean, and how is it different from että?

Jotta introduces a purpose or goal: “so that, in order that”.

…, jotta en unohda sitä.
= … so that I don’t forget it.

Että is a more general “that” for content clauses:

Tiedän, että määräaika näkyy kalenterissa.
= I know that the deadline is visible in the calendar.

In purpose clauses like “so that I don’t forget it”, jotta is the standard and most natural choice.
Using että there would sound off or nonstandard in written Finnish.

Why is there a comma before jotta?

In Finnish, you normally put a comma between a main clause and a subordinate clause, even if there is no pause in speech:

Määräaika näkyy kalenterissa punaisella, jotta en unohda sitä.
Main clause: Määräaika näkyy kalenterissa punaisella
Subordinate clause: jotta en unohda sitä

So the comma is there simply because jotta introduces a subordinate clause of purpose.

Why is it en unohda and not something like en unohda sitä tulevaisuudessa / a future tense? English uses “won’t forget”.

Finnish generally does not have a separate future tense. The present tense is used for:

present time
future time (when context makes it clear)

So:

jotta en unohda sitä
literally: so that I don’t forget it
but contextually: so that I won’t forget it (in the future).

If you tried to add something like a literal “will” tense, it would be ungrammatical; Finnish simply uses the present here.

Why is the object sitä (partitive) and not sen (genitive/accusative)?

Unohtaa (to forget) can take its object in different cases depending on aspect and polarity:

In affirmative, with a complete, definite object, you often use genitive/accusative:
Muistan sen. – I remember it.
Under negation with ei, the object is typically in the partitive:
En muista sitä. – I don’t remember it.
En unohda sitä. – I won’t forget it.

So here:

en (negative) + unohda → object goes into partitive: sitä.

Using sen in this exact sentence would sound wrong to native speakers.

Could I drop the sitä and just say jotta en unohda?

Yes, you can:

Määräaika näkyy kalenterissa punaisella, jotta en unohda.

This is grammatically fine and would be understood as “…so that I don’t forget (it/the deadline).”
Finnish often omits pronouns when the reference is clear from context.

Including sitä simply makes the sentence slightly more explicit, but it’s not strictly necessary here.

What exactly does sitä refer to? Why not repeat määräaika?

Sitä is a pronoun referring back to määräaika:

määräaika (noun) → third-person pronoun se
object form under negation → sitä (partitive)

Repeating the noun:

… jotta en unohda määräaikaa.

is also grammatically correct and normal. Here määräaikaa is in the partitive for the same reason (negation).

So you have two natural options:

jotta en unohda sitä.
jotta en unohda määräaikaa.

Why is the word order Määräaika näkyy kalenterissa punaisella and not, for example, Kalenterissa määräaika näkyy punaisella?

Finnish word order is fairly flexible. The default order here is:

Topic / what we’re talking about: Määräaika
Verb: näkyy
Where it’s visible: kalenterissa
How/with what color: punaisella

You can say:

Kalenterissa määräaika näkyy punaisella.

This is also grammatical. It just slightly emphasizes kalenterissa (in the calendar) as the starting point of the information. The original version treats määräaika as the main starting point.

What is the difference between näkyy kalenterissa punaisella and something like on merkitty kalenteriin punaisella?

Both can be used, but they emphasize different aspects:

Määräaika näkyy kalenterissa punaisella
= The deadline appears / is visible in the calendar in red.
Focus: the current visible state.
Määräaika on merkitty kalenteriin punaisella
= The deadline has been marked in the calendar in red.
Focus: the action of marking it there (someone marked it).

So näkyy is about what you see now; on merkitty is about what has been done (marked).

Is it obvious that kalenterissa here means “in my calendar”, not just some random calendar?

In Finnish, possessive pronouns (my, your, his/her) are often omitted when obvious from context.

kalenterissa = in the calendar
In a typical real-world situation (talking about your own schedule), people will naturally understand it as “in my calendar”.

If you want to make it explicit, you can say:

kalenterissani = in my calendar
minun kalenterissani = in my calendar (even more explicit)

But in many everyday sentences, kalenterissa alone is enough and sounds more natural.