Learn Hardcore Finnish: Kun sataa, koira pysyy sisällä. - When it rains, the dog stays inside.

In Finnish, weather verbs like sataa (to rain), tuulee (it’s windy), pyryttää (it’s snowing heavily) are typically subjectless/impersonal.
So sataa literally means (it) rains, but Finnish usually doesn’t include a dummy subject like English it.

What form is sataa? Is it an infinitive?

Sataa here is the 3rd person singular present tense of the verb sataa (to rain).
For many verbs (including this one), the dictionary form (infinitive) and the 3rd person singular present can look the same, so you identify it by context. In Kun sataa, ..., it’s clearly functioning as a finite verb: when it rains.

Why is there a comma: Kun sataa, koira pysyy sisällä.?

Finnish normally uses a comma after a subordinate clause when it comes first.
Kun sataa is a subordinate time clause (when it rains), so it’s followed by a comma before the main clause koira pysyy sisällä.

What exactly does kun mean here—could it also mean “when” in the sense of “once”?

Yes. Kun is commonly used for when in time clauses. Depending on context, it can mean:

when/whenever (general situation): Kun sataa, koira pysyy sisällä.
when (at the time that) (a specific event): Kun sataa huomenna, ... (When it rains tomorrow, ...)

In this sentence, it’s most naturally read as a general/habitual meaning: whenever it rains.

Could I use jos instead of kun?

Often yes, but there’s a nuance:

kun = when/whenever (treats raining as a time situation that happens)
jos = if (more conditional: maybe it rains, maybe not)

So:

Kun sataa, koira pysyy sisällä. = When/whenever it rains, the dog stays inside.
Jos sataa, koira pysyy sisällä. = If it rains, the dog stays inside. (more like a condition)

Why is koira in the basic form (not a case ending)?

Koira is the subject of the main clause, so it’s in the nominative (the basic dictionary form).
The verb pysyy agrees with it in person/number: koira pysyy = the dog stays.

What is the verb pysyy and what is its base form?

Pysyy means stays / remains. Its dictionary form is pysyä (to stay, to remain).
Conjugation here:

pysyä → pysyy (3rd person singular present)

So koira pysyy = the dog stays.

Why is the present tense used—does it mean present time only?

Finnish present tense often covers:

habitual/general truths (like English “When it rains, ...”)
near future in many contexts

Here it’s a general rule/habit, not necessarily “right now.”

What case is sisällä? Why not sisään or sisälle?

Sisällä means (being) inside—a location/state, not motion. It’s an adverb-like form historically related to the adessive (-lla/-llä), and it functions as a “where” word: inside.

Compare:

sisällä = inside (being there)
sisään = (going) in(to) inside (movement inward)
sisälle = (going) inside (to the inside area) (movement toward inside)

So pysyy sisällä = stays inside (no movement).

Can the word order change, like Koira pysyy sisällä, kun sataa?

Yes. Both are natural:

Kun sataa, koira pysyy sisällä. (sets the scene first)
Koira pysyy sisällä, kun sataa. (states the main point first)

The meaning stays basically the same; the difference is mostly information focus/style.

How would I say it in the past: “When it rained, the dog stayed inside”?

You’d typically use past tense in both clauses:

Kun satoi, koira pysyi sisällä.
Here satoi is past of sataa, and pysyi is past of pysyä.