Kirjoitan osoitteen muistilapulle, etten unohda sitä.

Finnish present tense commonly covers:

• right now: Kirjoitan osoitteen… = I’m writing the address…
• near future / intention (especially with a clear context): Kirjoitan osoitteen… = I’ll write / I’m going to write the address… Finnish often doesn’t need a separate future tense; the present does the job.

osoitteen is the total object form (often called “accusative” in learner materials, though it looks like the genitive singular -n for nouns). It’s used because the action is seen as complete: you write down the whole address (a finished result), not just “some of it.”

Compare:

• Kirjoitan osoitteen. = I write down the (whole) address.
• Kirjoitan osoitetta. = I’m writing the address / some of the address (process/unfinished/partial).

muistilapulle is allative (-lle), which often means onto / to / on a surface or target. So muistilapulle is “onto a note (note paper) / on a sticky note”.

You’ll often see:

• pöydälle = onto the table
• lapulle = onto a slip of paper / on a note

Sometimes, yes, but it changes the “spatial feel”:

• muistilapulle (allative) = onto/on a note (natural for writing on paper)
• muistilappuun (illative) = into the note (less typical for writing; illative is more “into” an enclosed space) For writing, -lle is the normal choice.

etten is a contracted form of että + en:

• että = that / so that
• en = I do not

So etten unohda literally corresponds to so that I don’t forget.

Finnish commonly merges että/ettei with a negative verb:

• etten = that/so that I don’t (1st person singular)
• ettet = that you don’t (2nd person singular/plural depending on context)
• ettei = that (someone) doesn’t / so that (it) doesn’t (very common general form)

In Finnish negative clauses, the main verb is in a special form called the connegative (it doesn’t take the personal ending). So you get:

• minä unohdan = I forget (affirmative, personal ending -n)
• minä en unohda = I don’t forget (negative verb en
  • connegative unohda)

The same pattern applies here:

• etten unohda = so that I don’t forget

With unohtaa (to forget), the object is very commonly in the partitive:

• unohtaa se → typically unohtaa sitä in real usage

So:

• etten unohda sitä = so that I don’t forget it

Using sen is possible in some contexts, but it often sounds more specific/totalizing or stylistically different; for a learner, treating unohtaa + partitive as the default is safest.

Finnish uses a comma to separate a main clause from a subordinate clause introduced by words like että (and forms like etten):

• Kirjoitan osoitteen muistilapulle, etten unohda sitä. Main clause: Kirjoitan osoitteen muistilapulle Subordinate purpose clause: etten unohda sitä

Yes—this is a purpose (“so that…”) construction. You can also use jotta:

• Kirjoitan osoitteen muistilapulle, jotta en unohtaisi sitä.

Difference in form:

• With etten, you typically get the straightforward negative clause: etten unohda…
• With jotta, Finnish often uses the conditional in careful standard language: en unohtaisi (wouldn’t forget), though in speech you may hear simpler variants too.

The neutral order is:

• Kirjoitan (verb)
• osoitteen (object)
• muistilapulle (where/onto what) Then the purpose clause.

You can reorder for emphasis:

• Muistilapulle kirjoitan osoitteen, etten unohda sitä. (emphasizes onto the note)
• Osoitteen kirjoitan muistilapulle… (emphasizes the address)

But the given sentence is the most neutral, “textbook-normal” phrasing.

Key points:

• Finnish stress is usually on the first syllable: O-soi-teen, ET-ten
• Double consonants are longer: etten has a long tt (hold it slightly longer than a single t)
• osoitteen: the -teen part is long ee (written ee), so it’s held longer than a single e.