Learn Hardcore Finnish: Tänään helle on kova, joten minä pysyn sisällä. - Today the heat is intense, so I stay inside.

Helle is a noun meaning heatwave / very hot weather (usually around 25–30°C or more, depending on context). It refers to a period or condition of hot weather, not just the temperature of one object.

Kuuma is an adjective meaning hot. You use it to describe how hot something is:

Vesi on kuumaa. – The water is hot.
Tänään on kuuma. – It is hot today.

In the sentence:

Tänään helle on kova – Today the heatwave is intense / the heat is severe.

Here, helle is treated as a “thing” (a weather condition) and described with an adjective (kova).

Why is it “helle on kova” and not something like “helle on kuuma”?

Literally, kova means hard, tough, strong, but it also works very naturally in Finnish to mean intense / severe in certain expressions.

So:

helle on kova ≈ “the heat is intense / severe”
sade on kova ≈ “the rain is heavy”
tuuli on kova ≈ “the wind is strong”

You could say helle on kova or tänään on kova helle, but helle on kuuma sounds odd because helle already means “hot weather”; saying it is “hot” again is redundant or unidiomatic. Instead, Finns say the helle is kova (intense) or kova helle (a strong heatwave).

Could I also say “Tänään on kova helle” or “Tänään on hellettä”?

Yes, both are possible, with small nuances:

Tänään on kova helle.
Very natural. Literally “Today (there) is strong heatwave.” This sounds very idiomatic.
Tänään on hellettä.
Uses partitive (hellettä) to say there is “(some) heatwave/very hot weather” today. This is also fine and common in weather-talk.

Your original:

Tänään helle on kova.
Grammatically fine, with a slightly more “descriptive” feel (you’re pointing to helle as a known condition and saying it’s kova today).

All three can be understood; usage preference depends on style and emphasis.

Why is the conjunction joten used here, and how is it different from koska or siksi?

In the sentence:

Tänään helle on kova, joten minä pysyn sisällä.

joten means “so / therefore” and introduces a result or consequence:

“Today the heat is intense, so I stay inside.”

Compare:

koska = because, introduces the reason:
- Minä pysyn sisällä, koska helle on kova.
  – I stay inside because the heat is intense.
siksi = for that reason / that’s why, usually followed by normal word order:
- Helle on kova. Siksi pysyn sisällä.
  – The heat is intense. That’s why I stay inside.

So:

joten → “so / therefore” (leads to a result clause)
koska → “because” (leads to a cause clause)
siksi → “for that reason / therefore” (adverb standing on its own)

Do I really need the pronoun minä here, or can I just say “… joten pysyn sisällä”?

You can absolutely drop minä:

Tänään helle on kova, joten pysyn sisällä.

Finnish usually omits subject pronouns because the verb ending already shows the person:

pysyn (I stay)
pysyt (you stay)
pysyy (he/she/it stays)

So minä pysyn is not wrong, but pysyn alone is more typical, unless you want to emphasize I (as opposed to someone else):

Minä pysyn sisällä, mutta sinä voit mennä uimaan.
– I will stay inside, but you can go swimming.

Why is the verb pysyä used, and how is it different from olla or jäädä in this context?

pysyä means to stay / to remain (not to leave, not to move away).

pysyä sisällä – to stay inside
pysyä rauhallisena – to stay calm

Comparisons:

olla = to be
- Olen sisällä. – I am inside. (Just stating location, not highlighting “staying”.)
jäädä = to stay / to remain behind / to be left (often about the moment of deciding to stay or being left somewhere)
- Jään sisälle. – I (will) stay inside / I’m staying inside (I don’t go out).
- Focus is often on the decision point or on not going with others.
pysyä = to continue to be in some place or state
- Pysyn sisällä. – I stay/remain inside (I keep myself inside, I don’t go out).

In your sentence, pysyn sisällä nicely expresses the idea “I will remain indoors”.

Why is it sisällä and not sisään or sisässä?

All three exist, but they’re used differently:

sisällä
- Adverb meaning inside / indoors (state).
- Used with pysyä, olla, etc., to show where you are/stay:
  - Olen sisällä. – I am inside.
  - Pysyn sisällä. – I stay inside.
sisään
- Directional adverb meaning (to) inside / in (movement into a space).
- Used with mennä, tulla, etc.:
  - Menen sisään. – I go inside.
  - Tule sisään! – Come in!
sisässä
- Also literally “inside (in)”, but usually for something being physically inside another thing, often more concrete or technical than sisällä.
- Akussa on jotain vikaa sisässä. – There is something wrong inside the battery.
- For “staying indoors”, sisällä is the natural choice.

So:

pysyn sisällä – I stay inside (location/state)
menen sisään – I go in (movement)

What case ending is in sisällä, and what does it normally mean?

sisällä historically uses the adessive ending -lla / -llä, which usually means “on / at”:

pöydällä – on the table
asemalla – at the station

But some words form fixed adverbs with this ending, where the meaning has shifted:

kotona (inessive-type) – at home
ulkona – outside
sisällä – inside / indoors

So grammatically sisällä is like “on/at the inside (area)” but in practice you just learn it as the basic adverb for “inside / indoors” in sentences like pysyä sisällä, olla sisällä.

Why is the verb pysyn in the present tense if it refers to what I will do today?

Finnish often uses the present tense for near future plans or decisions, especially when the time is already clear from context:

Huomenna menen Helsinkiin. – Tomorrow I’m going to Helsinki.
Illalla katson elokuvan. – This evening I’ll watch a movie.

Here:

Tänään helle on kova, joten pysyn sisällä.
– Today the heat is intense, so I (will) stay inside.

The word tänään (“today”) already places the action in time, so the present tense is natural and does not sound like a “general habit”; it refers to today’s plan.

Why is Tänään at the beginning? Could I say “Helle on kova tänään” instead?

Yes, you can say:

Helle on kova tänään, joten pysyn sisällä.

Both orders are grammatical. The difference is in emphasis / information structure:

Tänään helle on kova…
- Emphasizes today as the frame for what you are saying. (“Today the heat is intense …”)
Helle on kova tänään…
- Emphasizes helle more; “The heat is intense today (as opposed to other days).”

Finnish word order is flexible, and time adverbs like tänään, huomenna, nyt often appear at the start of the sentence to set the time context.

There’s no word for “the” in helle. How do I know it’s “the heat” and not “a heat”?

Finnish does not have articles (the, a/an). Nouns like helle are neutral, and English speakers must decide from context whether to translate them with “the” or “a”.

In weather expressions, English typically uses “the heat” or just “it’s hot” rather than “a heat”, so we translate:

Tänään helle on kova. → “Today the heat is intense.”

But in Finnish, helle itself doesn’t say “the” or “a”. Context tells you which English article sounds natural.