Learn Hardcore Finnish: Minä laitan villapaidan päälle, koska ulkona on kylmä. - I put a wool sweater on because it is cold outside.

In Finnish, verbs change their ending to match the subject (person and number). The dictionary form laittaa is the basic form (the infinitive), but in a sentence you usually need a conjugated form.

laittaa = to put (infinitive)
minä laitan = I put / I am putting

Conjugation of laittaa (present tense):

minä laitan – I put
sinä laitat – you put (singular)
hän laittaa – he/she puts
me laitamme – we put
te laitatte – you put (plural / formal)
he laittavat – they put

So in the sentence, laitan is the 1st person singular form that matches minä (I). Using minä laittaa would be ungrammatical, because the verb ending must agree with the subject.

Do I need to say Minä here, or can I just say Laitan villapaidan päälle, koska ulkona on kylmä?

You can absolutely drop minä. In Finnish, the subject pronoun is often omitted because the verb ending already shows who is doing the action.

Minä laitan villapaidan päälle… – grammatically correct
Laitan villapaidan päälle… – also correct and very natural

You usually include minä only when you want to:

emphasize the subject: Minä laitan villapaidan päälle (I, as opposed to someone else)
or in very clear, simple learner-style speech

In everyday Finnish, Laitan villapaidan päälle, koska ulkona on kylmä sounds perfectly normal and probably more common.

What is the infinitive (dictionary form) of laitan, and what does laittaa usually mean?

The infinitive (basic dictionary form) of laitan is laittaa.

Basic meanings of laittaa:

to put, to place (something somewhere)
to prepare food (colloquial: laittaa ruokaa = to cook / make food)
to set, to arrange

In this sentence, laittaa is being used in a common clothing expression laittaa [vaate] päälle = to put [a piece of clothing] on. So Minä laitan villapaidan päälle = I put a wool sweater on.

Why is it villapaidan and not villapaita?

Villapaita means wool sweater in its basic (nominative) form.
Here it appears as villapaidan, which is the genitive/accusative form.

The reason: it is the object of the verb laitan, and it is a total object (you are putting on the whole sweater, and the action is complete). With a total object in a normal affirmative sentence, Finnish usually uses the genitive form.

nominative: villapaita (a sweater)
genitive/accusative: villapaidan (the sweater, as a total object)

Rough pattern:

Laitan villapaidan päälle. – I’m putting the whole sweater on.
Laitan villapaitaa päälle. – would sound like you’re in the process of putting it on / not completing the action (partitive object; less common here, but possible with a slightly different nuance).

So villapaidan marks it as a definite, complete object of the action.

What exactly does päälle mean here, and why do we need it with clothing?

Päälle is a postposition meaning onto / on (with a sense of movement onto a surface or onto the body).

The pattern laittaa X päälle literally is:

laittaa – to put
villapaita – wool sweater
päälle – onto (oneself)

So literally: I put the wool sweater onto (myself).
Finnish doesn’t need to say on myself because päälle with a clothing object is understood as putting it on your body.

Common clothing expressions:

laittaa takki päälle – to put a coat on
laittaa kengät jalkaan – to put shoes on (here jalkaan = onto the foot/feet)
ottaa villapaita pois päältä – to take a sweater off (literally: off from on-top-of)

So päälle is necessary; Minä laitan villapaidan alone would normally sound incomplete (you put the sweater… where?).

What is the difference between päälle and päällä?

They are related but not the same:

päällä = on, on top of (static location, no movement)
päälle = onto, on to (movement towards that position)

Compare:

Minulla on villapaita päällä.
– I have a wool sweater on (it is on me, statically).
Minä laitan villapaidan päälle.
– I put a wool sweater on (movement from not-on to on).

This is similar to English on vs. onto, or in vs. into.

In many English contexts you don’t distinguish them, but Finnish does.

Could I use pukea instead of laittaa for putting clothes on?

Yes, but the structure changes a bit.

laittaa [vaate] päälle = to put a piece of clothing on (very common, everyday)
pukea = to dress / to put clothes on (slightly more formal / a bit different focus)

With pukea, you often say:

pukea (someone)
- Äiti pukee lapsen. – The mother dresses the child.
pukea (vaate) päälle
- Minä puen villapaidan päälleni. – I put the wool sweater on (myself).

In casual speech, laittaa villapaidan päälle is extremely common and probably the most natural choice here. Pukea can sound more formal or be used in different stylistic contexts.

What does koska mean grammatically, and is the comma before it necessary?

Koska is a subordinating conjunction meaning because.
It introduces a reason clause:

…koska ulkona on kylmä. – …because it is cold outside.

About the comma:

In standard written Finnish, you normally put a comma before a subordinating conjunction like koska when it starts a subordinate clause after the main clause:
- Minä laitan villapaidan päälle, koska ulkona on kylmä.

So yes, that comma is standard and correct. In very informal writing you might sometimes see it omitted, but in correct Finnish it should be there.

Why is it ulkona and not something like ulko?

Ulkona is the inessive case of ulko (outside), and it functions here as an adverb meaning outside / outdoors.

Finnish uses local cases instead of prepositions like “in, at, on”.
The ending -na/-nä here works like “in a state/place”:

ulkona – outside, outdoors
sisällä – inside
kaupungissa – in the city

You would not normally use a bare form ulko in this sentence. Ulkona is the natural, fixed way to say outside in this kind of weather sentence.

So ulkona on kylmä literally: outside is cold.

Can I change the word order in ulkona on kylmä?

Yes, but the most neutral, typical word order for a weather-type statement is:

Ulkona on kylmä. – It is cold outside.

Other possibilities and their feel:

On kylmä ulkona.
– Grammatically possible, but sounds a bit odd or more marked; you’re foregrounding is cold first, then adding outside almost as an afterthought.
Kylmä on ulkona.
– Very marked / poetic or contrastive: It’s outside that is cold (implying maybe inside it’s not cold). Not normal for a simple statement.

So for normal speech, Ulkona on kylmä is the natural word order.

Where is the word “it” in ulkona on kylmä? Why doesn’t Finnish use a subject like English does?

Finnish usually does not use a dummy “it” subject in weather expressions. English says:

It is cold outside.

Finnish simply says:

Ulkona on kylmä. – Outside is cold.

There is no separate word for “it”; the structure is basically:

ulkona – outside (location)
on – is
kylmä – cold (adjective, predicative)

Similar patterns:

Sataa. – It is raining. (no “it”)
Tuulee. – It is windy / the wind is blowing.

So not having an explicit “it” is completely normal in Finnish.

Can I say something like Minä laitan villapaidan päälleni instead of päälle?

Yes, this is also correct, just slightly more explicit and a bit more formal/literary:

Minä laitan villapaidan päälle.
– I put a wool sweater on (on myself – understood from context).
Minä laitan villapaidan päälleni.
– I put a wool sweater on me (literally: onto my top/onto myself).

Here päälleni = päällä (on) + -ni (my), fused together meaning on me.

Both are grammatical. In everyday spoken Finnish, päälle alone with a clothing object is extremely common and fully clear, so päälle is more typical in normal conversation.

Is minä always the word for “I”, or is there a shorter spoken form?

Minä is the standard written form for I.

In spoken Finnish, you will very often hear:

mä (in most southern/standard colloquial speech)
mie (in some eastern/northern dialects)

So in everyday speech, people might say:

Mä laitan villapaidan päälle, koska ulkona on kylmä.

But in formal/written Finnish or in textbooks, minä is used. Since the verb ending already encodes the person, in real speech many people just say:

Laitan villapaidan päälle, koska ulkona on kylmä.

and skip the pronoun altogether.