Ripustan takkini naulakkoon ja laitan avaimenperän pöydälle, etten hukkaa avainta.

Finnish often marks possession with a possessive suffix instead of a separate word:

• takki = coat
• takkini = takki + -ni = my coat

Because the verb already tells who the subject is (ripustan = I hang up), Finnish usually doesn’t need minun (my) here.

Ripustaa is the dictionary (infinitive) form: to hang up.
Ripustan is the present tense, 1st person singular: I hang up / I’m hanging up.

Ripustin would be past tense: I hung up.

Naulakkoon is the illative case, used for movement into something (or into a “destination”):

• naulakko = coat rack
• naulakkoon = into/onto the coat rack (as the destination)

With many -o/-ö ending words, the illative is formed by lengthening the vowel:

• -o → -oon, -ö → -öön

Often, yes, and both can sound natural depending on what you picture:

• naulakkoon (illative) focuses on the rack as the “storage place/destination”
• naulakolle (allative) focuses more on “onto” something (like a surface)

With coat racks, naulakkoon is very common because you’re putting something “in/into” the rack area (onto its hooks).

That -n is the typical marker of a total object in a normal affirmative clause (often identical to the genitive form):

• laitan avaimenperän = I put the keychain (down) / I place the keychain (there)

If you used the partitive (avaimenperää), it would suggest an incomplete/ongoing action, or “some of” something, depending on context.

Because the keychain is being placed onto a surface:

• pöydälle = onto the table (allative -lle)

By contrast:

• pöydässä = in/on the table area (location: on/at the table)
• pöytään = into the table (inside it), which usually isn’t what you mean

Etten means so that I don’t / in order not to. It’s essentially:

• että en → contracted to etten

So this part:

• ..., etten hukkaa avainta. means: ..., so that I don’t lose the key.

Finnish negation uses a separate negative verb (en, et, ei, emme, ette, eivät). After it, the main verb appears in the connegative form (no personal ending):

• en hukkaa = I don’t lose
  Not en hukkaan

So in the sentence, etten already contains en, and hukkaa is the connegative form.

Because in Finnish, a direct object is typically partitive in negative clauses:

• etten hukkaa avainta = so that I don’t lose the key

In an affirmative version you’d normally get a total object:

• hukkaan avaimen = I lose the key (I end up losing it)

Negation strongly pushes the object to the partitive: avainta.

The verb endings already show the subject:

• ripustan = I hang up
• laitan = I put/place

So minä is usually unnecessary unless you want emphasis or contrast (like I as opposed to someone else).

Yes, normally. Finnish uses a comma to separate a main clause from a subordinate clause:

• Ripustan ..., ja laitan ..., etten ...

Here etten hukkaa avainta is a subordinate purpose clause (why you do it), so it’s set off with a comma.

A few common ones:

• Long vowels matter: naulakkoon has a long oo (held longer than a single o).
• Double consonants matter: takkini (kk), hukkaa (kk) — hold the consonant longer.
• Stress is usually on the first syllable: RÍpustan TÁkkini NÁulakkoon.