Learn Hardcore Finnish: Arkipäivinä pukeudun mieluummin villapaitaan. - On weekdays I prefer to dress in a sweater.

Base word: arkipäivä = weekday
Essive plural: arkipäivinä = as weekdays / during weekdays → used idiomatically for time: on weekdays
Structure: arkipäivä + i (plural) + nä (essive ending) → arkipäivinä

The essive is very commonly used for time expressions like maanantaina (on Monday), viikonloppuna (on the weekend), and here arkipäivinä (on weekdays).

Why is arkipäivinä plural instead of singular?

Because the sentence talks about what happens on weekdays in general, not on one specific weekday.

Singular essive: arkipäivänä = on a weekday (one weekday)
Plural essive: arkipäivinä = on weekdays (on weekdays in general)

Finnish often uses plural forms for regular, repeated times:

öisin – at nights
kesäisin – in summers
arkipäivinä – on weekdays

So arkipäivinä matches the idea of a habitual action that happens on all / most weekdays.

Could I say arkisin or arkipäivisin instead of arkipäivinä? Is there any difference?

Yes, you can, and the difference is small.

All of these are correct here:

Arkipäivinä pukeudun mieluummin villapaitaan.
Arkisin pukeudun mieluummin villapaitaan.
Arkipäivisin pukeudun mieluummin villapaitaan.

Nuance:

arkipäivinä – literally on weekdays; slightly more “noun-like” and neutral.
arkisin – very common adverb: on weekdays, on workdays (as opposed to weekends).
arkipäivisin – also an adverb; can feel a bit more formal or explicit (“on each weekday”).

In everyday speech, arkisin and arkipäivinä are both very natural.

What exactly does pukeudun mean, and how is it different from English wear?

Pukeudun is the 1st person singular of pukeutua, which means to get dressed, to dress oneself, or to put clothes on.

pukeudun ≈ I get dressed / I dress (myself)
In this sentence: pukeudun villapaitaan = I dress in / dress into a wool sweater.

Difference from English:

English often says “I wear a sweater” for a general habit.
Finnish tends to express the action of putting something on with pukeutua and illative case (into what you dress).
For the state of wearing something, Finnish uses other structures, like:
- Minulla on villapaita päällä. – I am wearing a sweater.
- Käytän villapaitaa. – I use/wear a sweater (habitually).

So pukeudun focuses on the getting dressed action, even if the English translation uses wear.

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

Villapaitaan is in the illative singular case, which often means into, to, into a state.

Base noun: villapaita = wool sweater
Illative singular: villapaitaan = into a wool sweater

The verb pukeutua regularly takes its clothing in the illative:

pukeutua takkiin – to dress in a coat
pukeutua mekkoon – to dress in a dress
pukeutua farkkuihin – to dress in jeans
pukeutua villapaitaan – to dress in a wool sweater

So the meaning is literally: On weekdays I (tend to) dress myself *into a wool sweater.*

How is the form villapaitaan built from villapaita?

The pattern is the regular illative of an -a noun.

Base: villapaita (wool sweater)
Illative singular ending: usually -Vn (vowel + n), where the final vowel is lengthened.

Steps:

Take villapaita.
Lengthen the last vowel a → aa and add n.
You get villapaitaan.

Other examples:

koira → koiraan (into a dog)
huone → huoneeseen (into a room)
tyttö → tyttöön (into the girl)

So villapaita → villapaitaan is the normal illative singular formation.

Why is it singular villapaitaan and not plural like villapaitoihin, even though English might say sweaters?

Finnish usually uses the singular when talking about one item of clothing that you put on your body, even when English uses a generic plural.

Pukeudun villapaitaan. – I (prefer to) dress in a wool sweater.
English could say “I prefer wearing sweaters”, but the idea is “I put on one sweater when I dress”.

The plural illative villapaitoihin would normally mean:

You are literally putting on several sweaters at once, or
You are somehow emphasizing multiple separate sweaters.

For a general clothing choice like in this sentence, singular villapaitaan is the natural and default option.

What does mieluummin mean here, and how is it used?

Mieluummin means rather / preferably and expresses a preference between options.

In the sentence:

pukeudun mieluummin villapaitaan ≈ I would rather dress in a wool sweater / I prefer to dress in a wool sweater.

Key points:

mieluummin is an adverb that modifies the verb pukeudun.
It implies a comparison: wool sweaters are preferred over something else (e.g. T‑shirts, shirts), even if that “something else” is not mentioned.

So it answers the question: If I have options, what do I prefer to dress in?

How is mieluummin related to mielellään and mieluiten? When do I use each?

All three are related and describe how willingly you do something:

mielellään – gladly, willingly (no comparison)
mieluummin – rather, more willingly, preferably (comparative)
mieluiten – most willingly, preferably above all (superlative)

Examples:

Käyn mielellään elokuvissa.
I like going to the movies / I go to the movies gladly.
Käyn mieluummin teatterissa kuin elokuvissa.
I would rather go to the theatre than to the movies.
Käyn mieluiten konserteissa.
Most of all, I like going to concerts.

In your sentence, mieluummin is appropriate because it’s about preferring one option over others.

Can I use enemmän instead of mieluummin to say I like sweaters more?

Not in this structure. Enemmän means more (in quantity or degree), and its use is different.

mieluummin = rather, preferably → expresses choice / preference
enemmän = more → expresses amount or strength

Correct uses of enemmän:

Pidän villapaidoista enemmän kuin t-paidoista.
I like sweaters more than T‑shirts.

But:

pukeudun enemmän villapaitaan is not natural Finnish for “I prefer wearing a sweater”.
To express preference with pukeutua, you should use mieluummin, as in the original sentence.

So for this type of “I would rather …” meaning, mieluummin is the right choice.

Is the word order fixed? Can I say Pukeudun arkipäivinä mieluummin villapaitaan instead?

The word order is flexible, and your alternative is correct.

All of these are grammatical:

Arkipäivinä pukeudun mieluummin villapaitaan.
Pukeudun arkipäivinä mieluummin villapaitaan.
Pukeudun mieluummin villapaitaan arkipäivinä.

Differences are mostly about emphasis and flow:

Starting with Arkipäivinä highlights the when as the starting point: As for weekdays…
Starting with Pukeudun makes the action the first focus.

Finnish allows adverbials (like arkipäivinä, mieluummin) to move around quite freely, as long as the sentence remains clear and natural-sounding, which all of these versions do.

Why is the present tense pukeudun used for a habitual action like “on weekdays I prefer to dress in a sweater”?

Finnish uses the present tense for both:

actions happening right now, and
habitual or general actions.

Context and adverbs show which one is meant.

Examples:

Juon kahvia nyt. – I am drinking coffee now.
Juon kahvia joka aamu. – I drink coffee every morning. (habit)

Similarly:

Pukeudun villapaitaan. – I’m getting dressed in a sweater (right now) – in the right context.
Arkipäivinä pukeudun mieluummin villapaitaan. – On weekdays I (habitually) prefer to dress in a sweater.

There is no special continuous form like English “I am -ing”; Finnish relies on the simple present plus context.

Do I ever need a reflexive pronoun with pukeutua, like pukeudun itse?

Normally, no. The reflexive meaning is already built into pukeutua.

pukea = to dress someone (else)
pukeutua = to dress oneself, to get dressed

So:

Pukeudun villapaitaan. – I dress (myself) in a wool sweater.
There is no need to add itse (myself) for the normal meaning.

You only add itse if you want to emphasize that you are doing it yourself, not someone else:

Lapsi pukeutuu jo itse. – The child already dresses themself.
Minä pukeudun itse, kiitos. – I’ll dress myself, thanks.

In your sentence, pukeudun mieluummin villapaitaan is complete and natural without itse.