Kirjoitan koodin muistilapulle, etten unohda sitä.

Because koodin is the object of kirjoitan (I write / I am writing), and here it refers to a complete, definite thing: the code.

In Finnish, a total object often appears in a form that looks like the genitive singular, ending in -n:

• Kirjoitan koodin = I write the code / I’m writing down the code
• Kirjoitan koodia = I write code / I’m coding / I’m writing some code

So koodin suggests a specific whole item is being written down, not just an unspecified amount of code.

Muistilapulle is the allative form of muistilappu (note, memo slip, sticky note, depending on context). The ending -lle often means onto, to, or for.

So:

• muistilappu = a note
• muistilapulle = onto the note / on the note

With verbs of writing, Finnish often uses this kind of destination idea:

• kirjoittaa paperille = to write on paper
• kirjoittaa taululle = to write on the board
• kirjoittaa muistilapulle = to write on a note

Even though English says on, Finnish here uses a case that historically means movement toward a surface or place.

Etten is a shortened combination of että + en.

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

So etten unohda sitä literally comes from että en unohda sitä.

In this sentence, it expresses purpose:

• so that I don’t forget it
• so I won’t forget it

This shortening is very common in Finnish:

• etten = että en
• ettet = että et
• ettei = että ei
• ettemme = että emme
• ettette = että ette
• etteivät = että eivät

Because Finnish negative clauses work differently from English ones.

Finnish uses:

1. a negative auxiliary verb, which carries the person
2. the main verb in a special short form called the connegative

So:

• unohdan = I forget / I will forget
• en unohda = I do not forget / I won’t forget

In your sentence, the negative verb is inside etten:

• etten unohda = that I do not forget / so that I don’t forget

So -n does not appear on unohda, because the person I is already shown by en inside etten.

Because the object of a negative verb is normally in the partitive in Finnish.

Compare:

• Unohdan sen = I forget it
• En unohda sitä = I do not forget it

So in your sentence:

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

This is one of the most important object rules in Finnish:
negative sentence -> object usually in the partitive

That is why se becomes sitä here.

Both can express purpose, but etten is very natural here.

Compare:

• Kirjoitan koodin muistilapulle, etten unohda sitä.
• Kirjoitan koodin muistilapulle, jotta en unohtaisi sitä.

The first one is simple, common, and idiomatic:
I write the code on a note so I don’t forget it.

The second can also work, but it is often a bit more formal or more explicitly purposive, and the verb form may also change depending on style.

So etten is a very normal choice in everyday Finnish for this kind of so that I don’t... meaning.

Not completely. Finnish word order is fairly flexible, but different orders sound more or less natural and can change emphasis.

The given sentence is very natural:

• Kirjoitan koodin muistilapulle, etten unohda sitä.

You could also hear:

• Kirjoitan muistilapulle koodin, etten unohda sitä.

That version puts slightly more focus on where it is being written.

Still, the original order is probably the most neutral and natural in many contexts:

1. verb
2. object
3. place/destination
4. purpose clause

It can mean both, depending on context.

Finnish present tense often covers both the simple present and the English progressive:

• kirjoitan = I write / I am writing

So here, depending on the situation, the sentence could correspond to:

• I write the code on a note so I don’t forget it
• I’m writing the code on a note so I don’t forget it

Finnish does not usually make that distinction as strictly as English does.

Because etten unohda sitä is a subordinate clause.

Finnish normally separates subordinate clauses from the main clause with a comma:

• Kirjoitan koodin muistilapulle, etten unohda sitä.

Main clause:

• Kirjoitan koodin muistilapulle

Subordinate purpose clause:

• etten unohda sitä

So the comma is standard Finnish punctuation here.

Yes. Muistilappu literally means something like memory note or reminder note, but in practice it can refer to a small note used to remember something.

Depending on context, English might translate it as:

• note
• memo
• sticky note
• reminder note

So the exact English word may vary, but the Finnish idea is simply a small written note meant to help you remember something.

It breaks down like this:

• Kirjoitan = I write / I am writing
• koodin = the code
• muistilapulle = onto/on a note
• etten = so that I don’t
• unohda = forget
• sitä = it

So grammatically, it is:

main clause + purpose clause

• Kirjoitan koodin muistilapulle
• etten unohda sitä

A useful pattern to remember is:

• Teen X:n, etten / ettei / ettemme... + verb
• I do X so that I don’t / he doesn’t / we don’t...

That makes this sentence a good example of how Finnish expresses purpose in a natural everyday way.