Learn Hardcore Finnish: Kesällä jää sulaa nopeasti, kun aurinko paistaa. - In the summer the ice melts quickly when the sun shines.

It does not mean “on the summer” even though -lla often means “on” in other contexts; with time expressions, it’s best to remember it as “in / during”.

Why is jää in this form? Shouldn’t it be something like jäätä or jään?

Here jää (ice) is the subject of the sentence, so it appears in the nominative case, which for singular nouns is usually the bare stem:

jää sulaa = the ice melts / ice melts

You might know other forms:

jäätä (partitive) – e.g. näen jäätä = I see (some) ice
jään (genitive) – e.g. jään pinta = the surface of the ice

But for a subject in a normal clause, you use nominative: jää.

Also, Finnish has no articles (“a / an / the”), so jää can mean “ice” in general or “the ice”, depending on context.

Why is the verb sulaa and not some other form like sulattaa?

Sulaa and sulattaa are related but not the same:

sulaa = to melt (by itself), intransitive
- jää sulaa = the ice melts
sulattaa = to melt (something), transitive
- aurinko sulattaa jään = the sun melts the ice

In the given sentence, the ice is simply melting on its own, so we need the intransitive verb sulaa.

If you rewrote the sentence to make the sun the “doer”, you would use sulattaa, e.g.:

Kesällä aurinko sulattaa jään nopeasti.
In summer the sun melts the ice quickly.

Why is the verb in the present tense (sulaa, paistaa) if in English we might say “melts” or “is melting”?

Finnish present tense covers several English uses:

General truths / habits
- Kesällä jää sulaa nopeasti, kun aurinko paistaa.
  = In summer, ice melts quickly when the sun shines.
Actions happening right now (often clear from context)
- Jää sulaa nopeasti!
  = The ice is melting quickly!

So the same Finnish form sulaa can correspond to English “melts”, “is melting”, or even “will melt” in some contexts.

In your sentence, it’s describing a general fact, so English normally uses the simple present: melts / shines.

How does nopeasti relate to nopea? Why do we use nopeasti here?

Nopea is an adjective meaning “fast, quick”.

Finnish often forms adverbs (like “quickly”) by adding -sti to the adjective stem:

nopea → nopeasti = quickly, fast
hidas → hitaasti = slowly
helppo → helposti = easily

In the sentence, we need an adverb to modify the verb sulaa (“melts”), so we use nopeasti:

jää sulaa nopeasti = the ice melts quickly

Using nopea here (jää sulaa nopea) would be ungrammatical, because nopea is an adjective and doesn’t fit the role of modifying the verb.

What does kun mean here, and how is it different from koska?

In this sentence, kun means “when” in a temporal sense:

kun aurinko paistaa = when the sun shines

Differences:

kun
- Primary meaning: “when” (time)
- Can also mean “because” in some contexts, especially in spoken language, but here it’s clearly about time.
koska
- Means “because” or “since” (reason)
- Sometimes can mean “when” in questions (Koska tulet? = When are you coming?), but not as a conjunction in a sentence like this.

If you used koska instead:

Kesällä jää sulaa nopeasti, koska aurinko paistaa.
= In summer, ice melts quickly *because the sun shines.*

So kun in your sentence answers “When does it melt quickly?”, not “Why?”

Why is there a comma before kun?

Finnish typically separates a main clause and a subordinate clause with a comma, even in places where English might not:

Main clause: Kesällä jää sulaa nopeasti
Subordinate clause (introduced by kun): kun aurinko paistaa

Rule of thumb:
Put a comma before conjunctions like kun, koska, että, jos when they begin a subordinate clause that follows the main clause:

Menin kotiin, koska olin väsynyt.
En tule, jos sataa.

If the subordinate clause comes first, you still usually have a comma after it:

Kun aurinko paistaa, jää sulaa nopeasti.

What exactly does paistaa mean? Does it only mean “to shine”?

Paistaa has several related meanings, depending on context:

The sun shining
- Aurinko paistaa. = The sun is shining.
To fry / bake / roast (food)
- Paistan munan. = I fry an egg.
- Paistoin pullaa. = I baked buns.

In your sentence, with the subject aurinko, it clearly means “to shine”.

Other possible verbs for the sun are loistaa (“to glow, to shine brightly”), but aurinko paistaa is the most common everyday expression.

How flexible is the word order? Can we move Kesällä or the clause with kun?

Finnish word order is quite flexible, especially for elements like time and place, which you can move for emphasis or style.

All of these are grammatical and mean essentially the same:

Kesällä jää sulaa nopeasti, kun aurinko paistaa.
Jää sulaa nopeasti kesällä, kun aurinko paistaa.
Kun aurinko paistaa, jää sulaa nopeasti kesällä.
Kun aurinko paistaa kesällä, jää sulaa nopeasti.

What changes is emphasis and flow, not the basic meaning.

However, the verb usually stays second or near the beginning of its clause, and the subject normally is close to the verb in neutral sentences:

jää sulaa (subject + verb) is a typical neutral order.

How do we know whether jää here means “ice” or “ice cream”?

The word jää can mean:

jää = ice
In compounds, jäätelö = ice cream (a different word)

On its own, jää almost always means ice, not ice cream. Ice cream is jäätelö, not jää.

So in this sentence:

Kesällä jää sulaa nopeasti, kun aurinko paistaa.

Native speakers will automatically understand jää as ice, not ice cream, especially since plain ice melting in the sun is a very typical image and the natural interpretation.