Learn Hardcore Finnish: Haluaisin sellaisen villapaidan kuin sinulla. - I would like a sweater like the one you have.

Haluan = I want (direct, neutral statement of desire).
Haluaisin = I would like / I would want (more polite, a bit softer or more hypothetical).

Using Haluaisin is very common when:

Making polite requests:
- Haluaisin kahvin. = I’d like a coffee.
Expressing a wish that is not yet reality:
- Haluaisin asua Suomessa. = I’d like to live in Finland.

So in this sentence, Haluaisin makes it sound more like a polite wish than a blunt demand.

What exactly is sellaisen? How is it formed and what does it mean?

Sellaisen is an inflected form of sellainen.

sellainen = such / that kind of
- Related to se (that) + the suffix -llainen (kind/type).
In the sentence it appears as sellaisen, which is the genitive/accusative singular form.

It functions like an adjective modifying villapaidan:

sellainen villapaita = a sweater of that kind / such a sweater
sellaisen villapaidan = that kind of sweater (as a whole object)

Because villapaidan is in the -n form (genitive/accusative as total object), sellainen has to agree with it and becomes sellaisen.

So sellaisen villapaidan literally is “a such-kind sweater” in the object form, i.e. a sweater like that.

Why is it villapaidan and not villapaita?

Villapaidan is the genitive/accusative singular of villapaita (wool sweater).

Finnish marks a complete, specific object with this -n form:

Haluaisin villapaidan.
= I would like (to have) a sweater (one whole sweater).
Haluaisin villapaitaa.
= I would like (some) sweater / I feel like having a sweater (more vague, partitive).

In your sentence, the speaker wants one specific kind of whole sweater (like the one the other person has), so the object is a total object, and the form is villapaidan.

Because villapaita is the head noun of the object phrase, the modifier sellainen must match it: sellaisen villapaidan.

What does villapaita literally mean, and why is it written as one word?

Villapaita is a compound noun:

villa = wool
paita = shirt / sweater / top (context decides)

Together villapaita = wool sweater / woolly jumper.

Finnish usually writes compounds as one word when the parts together form a single concept:

käsilaukku (hand + bag) = handbag
aurinkolasit (sun + glasses) = sunglasses

So villapaita is one lexical item, and then it is inflected as a whole:

nominative: villapaita
genitive/accusative singular: villapaidan
partitive singular: villapaitaa, etc.

What does kuin mean here? Is it “than” or “like/as”?

Kuin is a comparison word that can translate both:

than (after comparatives):
- Hän on pidempi kuin minä. = He is taller than me.
as / like (in equality/similarity structures):
- Minulla on samanlainen villapaita kuin sinulla.
  = I have the same kind of sweater as you (do).

In sellaisen villapaidan kuin sinulla, kuin is used in the “like/as” sense:

Literally: “such a sweater as (there is) with you”
Natural English: “a sweater like the one you have” or “a sweater like yours”

So here kuin introduces the clause that specifies what kind of sweater is meant.

What case is sinulla, and why is it used instead of sinun?

Sinulla is the adessive case of sinä (you, singular):

sinä = you (nominative)
sinun = your (genitive)
sinulla = “on you / at you” (adessive)

Finnish expresses possession with a structure that literally means “on X is Y”:

Sinulla on villapaita.
Literally: “On you is a sweater.”
Meaning: You have a sweater.

In kuin sinulla, the full idea is:

kuin sinulla on = like the one you have

The verb on is often left out because it’s obvious from context:

sellaisen villapaidan kuin sinulla (on)
= such a sweater as you have

So sinulla appears because this comparison is really to “the sweater that is on you / that you have”, not to “your” as an adjective.

Could we say Haluaisin sellaisen villapaidan kuin sinulla on instead? Is there any difference?

Yes, that is completely correct:

Haluaisin sellaisen villapaidan kuin sinulla on. = I would like a sweater like the one you have.

Difference:

kuin sinulla
– Shorter, more colloquial, the on is understood.
kuin sinulla on
– More explicit, slightly more formal or careful speech/writing.

Meaning-wise they are the same; you are just choosing whether to say the on out loud or leave it implied.

Why isn’t it kuin sinun villapaita or kuin sinulla villapaita?

Those versions are not natural Finnish for this idea.

kuin sinun villapaita
- This would literally be “like your sweater”, using sinun as a possessive.
- But Finnish typically prefers the possessive structure with sinulla on rather than a bare “your + noun” in these comparison phrases.
- The natural pattern is:
  sellainen X kuin sinulla (on)
  not sellainen X kuin sinun X.
kuin sinulla villapaita
- This sounds incomplete or ungrammatical because the normal possession structure needs the verb:
  - Sinulla on villapaita. = You have a sweater.
- If you keep sinulla, you normally also have on, or you drop on only when everything else makes it clear (as in kuin sinulla).
- kuin sinulla villapaita breaks that pattern and doesn’t work idiomatically.

So the standard comparative pattern here is:

sellainen villapaita kuin sinulla (on)

Can the word order be changed? For example, could you say Haluaisin villapaidan sellaisen kuin sinulla?

You can move parts around, but some orders sound more natural than others.

Grammatically acceptable options:

Haluaisin sellaisen villapaidan kuin sinulla.
– Very natural and common.
Haluaisin villapaidan sellaisen kuin sinulla.
– Also understandable, but a bit heavier and less typical; you end up splitting the descriptor sellainen away from the comparison kuin sinulla.
Sellaisen villapaidan haluaisin kuin sinulla.
– Emphasis on sellaisen villapaidan (that kind of sweater). Sounds like you’re stressing what exactly you want.

The key:

Keep sellaisen close to villapaidan, because they belong together.
Keep the comparison kuin sinulla (on) right after the thing it describes.

So your proposed sentence is not wrong, but the original word order is smoother and more idiomatic.

Are there other natural ways to say “a sweater like yours” in Finnish?

Yes, several:

Using samanlainen (the same kind of):
- Haluaisin samanlaisen villapaidan kuin sinulla (on).
  = I’d like the same kind of sweater as you (have).
Using sinunlaisesi (your kind/type):
- Haluaisin sinunlaisesi villapaidan.
  = I’d like a sweater like yours / a sweater of your kind.
  (A bit more colloquial or expressive.)
More direct possessive comparison:
- Haluaisin villapaidan, joka on samanlainen kuin sinun.
  = I’d like a sweater that is the same as yours.
  (Here sinun stands for sinun villapaitasi.)

The original:

Haluaisin sellaisen villapaidan kuin sinulla.

is very natural and is probably the most typical everyday way to express “I’d like a sweater like the one you have.”

Why do sellaisen and villapaidan both end in -n?

Because they are both in the same case, functioning together as a total object phrase.

villapaita (wool sweater) → villapaidan (genitive/accusative singular)
sellainen (such / that kind of) → sellaisen (genitive/accusative singular)

In Finnish, adjectives and pronouns that modify a noun must agree with it in:

number (singular/plural)
case

So:

Noun alone: villapaidan
With modifier: sellaisen villapaidan

Both take -n because the speaker means one whole, specific sweater, and the conditional verb haluaisin is taking that whole object.