Learn Hardcore Finnish: Kirjoitan henkilötunnuksen muistilapulle, etten unohda sitä. - I write the personal identity code on a sticky note so that I don’t forget it.

Kirjoitan is the verb kirjoittaa (to write) in the 1st person singular present tense: I write / I am writing.
Finnish present tense often covers both English simple present and present continuous, so the context decides.

Why is there no word for I (like minä) in the sentence?

Finnish usually drops the subject pronoun because the verb ending already shows the person: kirjoita-n = I write.
You could add minä for emphasis/contrast (e.g., I am writing it, not someone else), but it’s not needed here.

What case is henkilötunnuksen, and why does it end in -n?

Henkilötunnuksen is the object henkilötunnus (personal identity code) in the genitive-looking form -n, which is commonly used for a total object in the singular.
Here you’re writing down the whole thing (a complete, definite item), so Finnish treats it as a total object → henkilötunnuksen.

So is that -n ending genitive or accusative?

Form-wise it looks like genitive, but functionally it’s serving as the accusative/total object in a typical Finnish pattern.
In many grammars you’ll see this described as:

singular total object often appears as -n (same form as genitive)
except personal pronouns (which have a distinct accusative form like minut, sinut, etc.)

Why is muistilapulle used, and what does -lle mean?

Muistilapulle = muistilappu (note, memo slip) + -lle (the allative case).
Allative often means onto or to a surface/target: onto a note / on a piece of paper.
So Kirjoitan … muistilapulle is literally I write … onto a note.

Could I say muistilappuun instead of muistilapulle?

Yes, but the nuance changes slightly:

muistilapulle (allative) = onto/on a note (treating it like a surface/medium)
muistilappuun (illative) = into the note (a common choice too, focusing on putting the content “into” it)
Both can be heard; -lle is very natural for writing something onto a note.

What does etten mean, and how is it formed?

Etten means so that I don’t / in order that I won’t.
It’s essentially että (that / so that) + the negative verb en (I don’t) merged into one word:

että + en → etten
This is very common Finnish in purpose clauses.

Why is it unohda and not unohdan after etten?

In Finnish negative sentences use: 1) a negative auxiliary (here en inside etten) that shows the person
2) the main verb in the connegative form (a special form without the personal ending)

So:

affirmative: unohdan = I forget
negative: en unohda = I don’t forget
Here it’s embedded: etten unohda = so that I don’t forget.

Why is sitä in the partitive case?

Because the clause etten unohda sitä is negative, and in Finnish a direct object in a negative clause is very often in the partitive.
Compare:

Muistan sen. = I remember it. (non-negative → total object)
En muista sitä. = I don’t remember it. (negative → partitive)

So sitä is the partitive form of se (it/that).

Why doesn’t the pronoun match the earlier object case? (First henkilötunnuksen, then sitä)

They’re in different clauses with different grammar:

Main clause (affirmative): Kirjoitan henkilötunnuksen … → total object (henkilötunnuksen)
Purpose clause (negative): etten unohda sitä → negative clause → partitive object (sitä)

So the case changes because the polarity (affirmative vs negative) changes.

What is the function of the comma before etten?

The comma separates the main clause from the subordinate clause:

Kirjoitan … muistilapulle, (main clause)
etten unohda sitä. (purpose clause)

Finnish uses commas quite regularly to mark clause boundaries like this.

Is etten unohda sitä the only way to express “so that I won’t forget it”?

It’s one very common way. You can also see:

jotta en unohda sitä = also so that I don’t forget it
ettei is another merged form (roughly että + ei), often used especially when the subject isn’t 1st person singular, but etten is the correct merged form for I don’t here.