Learn Hardcore Finnish: Äkkiä juoksen sisään, koska sataa. - Suddenly I run inside because it is raining.

juoksen is 1st person singular (I), present tense of juosta (to run).
Because that ending -n clearly marks I, the separate pronoun minä is usually unnecessary.

You can say:

Äkkiä minä juoksen sisään, koska sataa.

This sounds more emphatic on I (as in: I am the one who runs inside, not someone else). In neutral sentences, Finns generally drop minä.

What exactly is the form juoksen, and how is it formed from juosta?

juoksen is:

verb: juosta (to run)
tense: present
person: 1st person singular
form: indicative (normal statement)

The full present-tense paradigm of juosta is:

(minä) juoksen – I run
(sinä) juokset – you run (singular)
hän juoksee – he / she runs
(me) juoksemme – we run
(te) juoksette – you run (plural / formal)
he juoksevat – they run

The English progressive I am running is usually just a simple present in Finnish: juoksen covers both I run and I am running, depending on context.

Why is Äkkiä at the beginning of the sentence? Could it also come later?

Yes, it can move. Word order in Finnish is flexible and often used for emphasis.

Äkkiä juoksen sisään, koska sataa.
– Emphasis on how you act: quickly / suddenly I run inside.
Juoksen äkkiä sisään, koska sataa.
– More neutral; juoksen sisään is kept together, äkkiä just modifies it.
Juoksen sisään äkkiä, koska sataa.
– Also possible; a bit heavier, with a slight afterthought feel.

Putting Äkkiä first makes the suddenness / quickness the first thing the listener notices. Finnish often front-loads what is important or new.

What does Äkkiä mean exactly, and how is it different from nopeasti?

Both are adverbs related to doing something fast, but the nuance is different:

Äkkiä = suddenly, quickly, in a hurry
- Often implies a sudden change or urgent reaction.
- In your sentence: you rush inside because it starts raining.
nopeasti = fast, quickly (more neutral)
- Describes speed without the same feeling of suddenness or surprise.

So:

Äkkiä juoksen sisään, koska sataa.
– Feels like: All of a sudden, I rush inside, because it is raining.
Juoksen nopeasti sisään, koska sataa.
– More like: I go inside at a fast speed, because it is raining. (less emotional)

What exactly is sisään, and how is it different from sisällä, sisältä, and sisälle?

These are direction / location adverbs related to being inside:

sisään = (to) inside, into
- Direction towards the interior.
- Used with verbs like mennä (go), tulla (come), juosta (run).
- juoksen sisään = I run inside / into (a room, building, etc.).
sisällä = inside, indoors (location, static)
- No movement; you are already inside.
- Olen sisällä. = I am inside.
sisältä = from inside (movement out of an interior)
- Tulen sisältä. = I am coming from inside.
sisälle = also (to) inside
- Often interchangeable with sisään.
- juoksen sisälle is very common in everyday speech; juoksen sisään is maybe slightly more neutral / written, but both are fine.

So in your sentence, juoksen sisään focuses on the movement into the inside of something.

Why is sisään a separate word and not just a case ending on some noun?

Finnish can express direction in two ways:

With a noun + case ending, e.g.
- Juoksen taloon. = I run into the house.
  - talo (house) + -on (illative) → taloon = into the house.
With a directional adverb like sisään, often when the place is clear from context or not specified:
- Juoksen sisään. = I run inside (into the building / inside somewhere).

In your sentence, the place is not specified, so Finnish just uses the adverb sisään. If you wanted to be explicit, you could say:

Äkkiä juoksen taloon sisään, koska sataa.
– I quickly run into the house, because it is raining.

Why is there a comma before koska? Is this the same as in English?

Yes, the rule is similar but applied more consistently.

koska introduces a subordinate clause (a dependent clause giving a reason). In Finnish:

When a subordinate clause follows the main clause, you normally put a comma before it.

So:

Äkkiä juoksen sisään, koska sataa.
– Main clause: Äkkiä juoksen sisään
– Subordinate clause: koska sataa

If you reverse the order, you still usually use a comma:

Koska sataa, juoksen äkkiä sisään.

In practice, Finnish uses commas more regularly than English for this type of structure.

Does koska always mean because, or can it also mean when?

koska has two main uses:

because (reason) – very common
- Äkkiä juoksen sisään, koska sataa.
  – I quickly run inside because it is raining.
when (in questions or certain contexts)
- Koska tulet? = When are you coming?
- En muista, koska se tapahtui. = I don't remember when it happened.

In your sentence, the structure ..., koska sataa clearly gives a reason, so it means because, not when. Context and sentence type (question vs statement) tell you which meaning is intended.

What is sataa grammatically, and why is there no Finnish word for it?

sataa is the verb sataa in 3rd person singular, present tense. The verb itself means to rain (or, more broadly, to precipitate).

Finnish weather verbs are usually impersonal:

Sataa. = It is raining.
Sataa lunta. = It is snowing (literally: it rains snow).
Tuulee. = It is windy / the wind is blowing.
Ukkostaa. = There is a thunderstorm / it is thundering.

There is no explicit subject like it. The verb is simply used in 3rd person singular with no pronoun:

koska sataa = because (it) rains / because (it) is raining.

So Finnish does not need a dummy subject it the way English does.

Can you say koska se sataa instead of koska sataa?

You can, but it changes the feel:

koska sataa
– Neutral, standard way to talk about rain: because it is raining.
koska se sataa
– Possible, but sounds more colloquial or emphatic, almost like talking about the rain as something already mentioned (that thing is raining).
– Often used in speech with a slightly emotional or narrative tone.

For clear, neutral Finnish, koska sataa is better in this sentence.

Could I say Juoksen sisään, koska sataa without Äkkiä? How does that change the meaning?

Yes, that is perfectly correct:

Juoksen sisään, koska sataa.
– I run inside because it is raining.

Leaving out Äkkiä simply removes the idea of suddenness / hurry. The core meaning (you go inside because of the rain) stays the same.

Conversely, you could focus even more on the suddenness:

Juoksen äkkiä sisään, koska alkaa sataa.
– I quickly run inside because it starts to rain.

How would I say this in a more colloquial spoken Finnish style?

In everyday speech, people often:

Use mä instead of minä.
Shorten words (e.g., nopeesti instead of nopeasti).
Shorten koska to ku.

A colloquial version might be:

Mä juoksen nopeesti sisään, ku sataa.

Or with äkkiä kept:

Mä juoksen äkkii sisään, ku sataa.

This is typical spoken Finnish; your original sentence is neutral / written standard.

How would I say I don’t run inside, although it is raining using a similar structure?

You can reuse most of the elements and change two things:

Use the negative verb for I: en juokse (I don’t run).
Use vaikka instead of koska, because now it is contrast, not reason.

Result:

En juokse sisään, vaikka sataa.
– I don’t run inside, although it is raining.

If you want to keep the quickness idea in the negative:

En juokse äkkiä sisään, vaikka sataa.
– I don’t quickly run inside, although it is raining.