Learn Hardcore Finnish: Sää voi muuttua nopeasti talvella. - The weather can change quickly in winter.

You almost never say a plural like säät in everyday speech for ordinary weather. Plural säät is only used in special contexts, like talking about weather types in different countries or long-term climate overviews, e.g.:

Euroopan säät ovat vaihtelevia. – The weathers (weather patterns) in Europe are variable.

So in this sentence, sää is just the normal way to say “the weather” as a singular noun.

What is voi here? Is it like “can” or “may” in English?

Voi is the 3rd person singular form of the verb voida, which means “can / may / to be able to / to be possible.”

Here, voi works like a modal verb, similar to English “can”:

Sää voi muuttua... – The weather can change... / The weather may change...

Depending on context, voi can express:

possibility: The weather may change.
typical ability/potential: The weather can (tends to be able to) change.

With sää, it’s usually about possibility or typical behavior, not permission.

Why is the verb muuttua and not muuttaa?

Finnish has two related verbs:

muuttua – to change (by itself), become different
- intransitive (no direct object)
muuttaa – to change (something), to move (house)
- transitive (takes a direct object)

In Sää voi muuttua nopeasti talvella, the weather is changing by itself, so you must use the intransitive verb:

Sää voi muuttua. – The weather can change (on its own).

If you used muuttaa, you’d be saying that the weather is changing something else:

Hän voi muuttaa suunnitelmaa. – He/She can change the plan.

So muuttua is the only correct choice in this sentence.

Why is it nopeasti and not nopea?

Nopea is an adjective: fast, quick.
Nopeasti is an adverb: fast / quickly.

In Finnish, many adverbs are formed from adjectives with the ending -sti:

nopea → nopeasti (fast → quickly)
hidas → hitaasti (slow → slowly)
selvä → selvästi (clear → clearly)

In this sentence, we’re describing how the weather can change, so we need an adverb:

muuttua nopeasti – to change quickly

Using nopea here (muuttua nopea) would be ungrammatical.

What case is talvella, and why does it mean “in winter”?

Talvella is in the adessive case.

Basic form: talvi – winter
Adessive singular: talvella – literally “on (the) winter”

The adessive ending is -lla / -llä. It often means “on” or “at”, but with time expressions, Finnish frequently uses the adessive where English uses “in” or “during”:

talvella – in winter
kesällä – in summer
yöllä – at night
viikonloppuna (another time form) – on the weekend

So talvella naturally translates as “in winter” or “during winter”, even though the literal case meaning is “on/at”.

Could the word order be different, like Talvella sää voi muuttua nopeasti?

Yes, that word order is perfectly natural:

Sää voi muuttua nopeasti talvella.
Talvella sää voi muuttua nopeasti.

They both mean the same in everyday use:
“The weather can change quickly in winter.”

The difference is in emphasis:

Starting with Sää focuses first on the weather.
Starting with Talvella puts more focus on the time frame (in winter), as in:
- In winter, the weather can change quickly (as opposed to other seasons).

Finnish word order is relatively flexible, especially with known elements, but the most neutral patterns keep the verb near the beginning and don’t separate verb and its key complements too much. Both versions are very common and correct.

Does this sentence talk about the present or the future? There’s no future tense, right?

Finnish does not have a separate future tense. The present tense is used for both present and future meanings, and context makes it clear.

Sää voi muuttua nopeasti talvella can mean:

General fact / habit:
The weather can change quickly in winter (it’s typical).
Future possibility:
The weather may change quickly in winter (this coming winter).

You don’t need a special verb form for the future; the basic form voi muuttua covers it.

How would I say “The weather can change very quickly in winter”?

You can add an adverb of degree before nopeasti. Common options:

Sää voi muuttua todella nopeasti talvella.
Sää voi muuttua hyvin nopeasti talvella.
Sää voi muuttua tosi nopeasti talvella. (more informal)

All of these mean roughly:

The weather can change very/really quickly in winter.

Todella and hyvin are a bit more neutral; tosi is more colloquial.

How would I make this sentence negative: “The weather cannot change quickly in winter”?

To make it negative, use the negative verb ei and put voida in its connegative form voi:

Sää ei voi muuttua nopeasti talvella.
The weather cannot change quickly in winter.

Structure:

sää – subject
ei – negative verb (3rd person singular)
voi – connegative form of voida
muuttua – infinitive
nopeasti talvella – adverbials

The main change from the affirmative is simply adding ei before voi.

Is there any difference between sää and ilma, since both can relate to weather?

Both sää and ilma can touch on weather, but they’re used differently:

sää – weather in general: rain, sun, temperature, etc.
- Sää on huono. – The weather is bad.
- Sää voi muuttua nopeasti.
ilma – primarily air, but can sometimes mean weather in older or more poetic style, or in set phrases:
- Ilma on kylmä. – The air is cold. (often also felt as “It’s cold outside.”)
- Ulkona on raikas ilma. – The air outside is fresh.

In your sentence, you must use sää.
Ilma voi muuttua nopeasti talvella would sound odd or at least much less natural in the meaning “the weather can change quickly in winter.”