Learn Hardcore Finnish: Onpa hauskaa, kun saman kynsilakan väri sopii myös hänen kampaukseensa. - How fun it is when the colour of the same nail polish also matches her hairstyle.

You talk about something non-specific, general, or abstract:
- On hauskaa = It is fun (in general / this situation is fun).
You’re describing an activity or state rather than a concrete, countable thing:
- On hauskaa, kun… = It is fun when… (general feeling about the situation).

If it were describing a specific, definite thing, you might see nominative hauska, e.g.:

Tämä elokuva on hauska. = This movie is funny.

But in this sentence, on(va) hauskaa refers to the experience or situation of the colors matching, so partitive hauskaa is natural.

What nuance does kun have here? Is it “when” or “because”?

In this sentence, kun introduces a subordinate clause that explains in what situation it is fun:

Onpa hauskaa, kun saman kynsilakan väri sopii myös hänen kampaukseensa.

Literally: It sure is fun, when the color of the same nail polish also suits his/her hairstyle.

Depending on context, kun can often be translated in a few ways:

Temporal / situational “when”:
- It’s fun *when this happens.*
Mildly causal “because / as”:
- It’s fun, *because the same nail polish color also suits the hairstyle.*

Both ideas (the situation and a gentle cause) are present here. In everyday English, you might translate it as:

It’s so nice *when the same nail polish color also suits your hairstyle.*

Note: if you used koska instead, it would sound more directly causal (because) and slightly less like a general “whenever/when” situation.

What does saman mean here, and why is it in that form?

Saman is the genitive singular of sama (same).

sama (nominative) = same (subject/basic form)
saman (genitive) = of the same

In saman kynsilakan väri, saman modifies kynsilakan (of the nail polish) and together they describe väri (color). Literally:

saman kynsilakan väri = the color of the same nail polish / the same nail polish’s color.

The genitive saman matches kynsilakan, which is also in genitive. This is a typical Finnish genitive chain: one genitive word modifies another genitive that modifies the head noun (väri).

Why is kynsilakan in that form? What case is it?

Kynsilakan is the genitive singular of kynsilakka (nail polish).

kynsilakka (nominative) = nail polish
kynsilakan (genitive) = of (the) nail polish

In saman kynsilakan väri:

väri = color (head noun)
kynsilakan väri = the color of the nail polish.

Genitive is used to show a possessive or “of”-relationship:

kirjan kansi = the book’s cover / cover of the book
kynsilakan väri = the nail polish’s color / color of the nail polish.

So saman kynsilakan väri = the color of the same nail polish.

Can you break down saman kynsilakan väri word by word with meanings?

Yes:

saman
- base: sama = same
- case: genitive singular → of the same
kynsilakan
- base: kynsilakka
  - kynsi = nail
  - lakka = varnish / lacquer
  - kynsilakka = nail polish
- case: genitive singular → of (the) nail polish
väri
- base: väri = color

Put together:

saman kynsilakan väri = the color of the same nail polish.

What does sopii mean here, and what structure does it use?

Sopii is the 3rd person singular present of sopia, which in this context means to suit, to match, to go well with.

The typical pattern here is:

jokin sopii johonkin
something suits / matches something (else)

The verb sopia often takes its “target” in the illative case (the “into/onto” case), which answers mihin? (into what, to what?).

In the sentence:

saman kynsilakan väri = subject (the color of the same nail polish)
sopii = suits / matches
hänen kampaukseensa = illative (to his/her hairstyle)

So:
saman kynsilakan väri sopii hänen kampaukseensa
= the color of the same nail polish suits his/her hairstyle.

Why is it hänen kampaukseensa and not just hänen kampaukseen?

Hänen kampaukseensa contains both a separate pronoun and a possessive suffix:

hänen = his/her (genitive pronoun)
kampaus = hairstyle
kampaukseen = illative of kampaus (into/to the hairstyle)
kampaukseensa = kampaukseen
- possessive suffix -nsA (to his/her hairstyle)

So:

hänen kampaukseen (without suffix) is usually considered incomplete or at least stylistically off.
Standard Finnish normally doubles possession:
- hänen kampauksensa = his/her hairstyle (nominative + suffix)
- hänen kampaukseensa = into/to his/her hairstyle (illative + suffix)

The pattern is:

hänen + [noun in case] + -nsA.

Examples:

hänen taloonsa = to his/her house
hänen kirjaansa = to his/her book

So hänen kampaukseensa correctly marks to his/her hairstyle with possession.

What exactly does the ending -nsa / -nsä in kampaukseensa do?

The -nsa/-nsä ending is a 3rd person possessive suffix. It corresponds to his/her/their when the possessor is in the 3rd person.

kampaus = hairstyle
kampauksensa = his/her/their hairstyle
kampaukseensa = to his/her/their hairstyle (illative + possessive suffix)

So:

hänen kampaukseensa = to his/her hairstyle
(hänen
- kampaukseensa, both marking the same possessor).

In modern standard Finnish, using both the pronoun (hänen) and the suffix (-nsa) is common and considered “complete”.

What is the function of myös here, and could it be placed elsewhere?

Myös means also / too / as well.

In the sentence:

… väri sopii myös hänen kampaukseensa.

it emphasizes that the nail polish color not only works in its primary role (on the nails) but also suits the hairstyle.

You can move myös around slightly, but word order affects emphasis:

väri sopii myös hänen kampaukseensa
Neutral: the color also suits his/her hairstyle.
väri sopii hänen kampaukseensa myös
A bit less common; myös now leans toward the whole phrase coming before it.
myös väri sopii hänen kampaukseensa
Now myös modifies väri more strongly: the color too suits his/her hairstyle (perhaps contrasting with something else that suits it).

The original position (sopii myös hänen kampaukseensa) is the most natural and neutral here.

Is kun saman kynsilakan väri sopii myös hänen kampaukseensa a full clause? How does it relate to Onpa hauskaa?

Yes, it is a subordinate clause introduced by kun:

Main clause: Onpa hauskaa (How fun it is).
Subordinate clause: kun saman kynsilakan väri sopii myös hänen kampaukseensa (when the color of the same nail polish also suits his/her hairstyle).

Together:

Onpa hauskaa, kun …
= It’s (really) fun (that/when) …

This is a common pattern where:

On(va) hauskaa, kun… ≈ It is fun when…
On kiva, että… ≈ It is nice that…

The comma between the clauses is mandatory in Finnish orthography in this kind of structure.

Could you rephrase the sentence in simpler Finnish while keeping the meaning?

One natural, slightly simpler rephrasing:

On hauskaa, että saman kynsilakan väri sopii myös hänen kampaukseensa.

Changes:

kun → että:
- että = that (more directly marking the following as a fact you’re commenting on).
The nuance shifts a bit from “it’s fun when this happens” toward “it’s fun that this is the case,” but in many contexts they’re very close.

You could also simplify the noun phrase slightly, depending on context, for instance:

On hauskaa, että kynsilakan väri sopii myös hänen kampaukseensa.
(It’s fun that the nail polish color also suits his/her hairstyle.)

dropping saman (the same) if it’s already clear from context what nail polish is being talked about.