Learn Hardcore Finnish: Sinun ei pidä juosta rappukäytävässä, ettei vahinko satu. - You must not run in the stairwell so that an accident doesn't happen.

Sinun ei pidä juosta = You should not / must not run.
- pitää
  - genitive pronoun + infinitive expresses obligation or recommendation:
    - Sinun pitää juosta. = You must run.
    - Sinun ei pidä juosta. = You must not / should not run.
Sinä et juokse. = You are not running / You do not run.
- This simply states a fact in the present; it does not express a rule or prohibition.

So the structure is:

[genitive subject] + (ei) + pitää + [infinitive] → someone (should / must / must not) do something.

Why is sinun in the genitive instead of the basic form sinä?

With the obligation-verb use of pitää, Finnish normally puts the “doer” into the genitive:

Minun pitää mennä. = I must go.
Sinun pitää mennä. = You must go.
Hänen pitää mennä. = He / She must go.

In the negative:

Minun ei pidä mennä. = I must not / should not go.
Sinun ei pidä juosta. = You must not / should not run.

So:

Sinä = nominative, used as the grammatical subject in normal clauses (Sinä juokset = You run).
Sinun = genitive, used in this construction [Genitive + pitää + infinitive] for obligation.

This is just a fixed, very common pattern with pitää in the sense “must / should”.

Why is the verb juosta in that form and not juokse?

Juosta is the basic infinitive form (1st infinitive). After pitää (in the sense of “must/should”), Finnish uses this infinitive:

Sinun pitää juosta. = You must run.
Sinun ei pidä juosta. = You must not / should not run.
Minun pitää opiskella. = I must study.
Meidän ei pidä meluta. = We must not make noise.

Juokse would be the imperative (a direct command):

Älä juokse rappukäytävässä! = Don’t run in the stairwell!

So:

pitää + juosta → “(someone) should/must run” (obligation).
juokse! → “run!” (command).

What exactly does rappukäytävässä mean, and how is it formed?

rappukäytävässä means “in the stairwell / in the staircase (corridor)”.

Breakdown:

rappu = stair, staircase
käytävä = corridor, hallway
Combined: rappukäytävä = stairwell, stair corridor
Ending -ssä = inessive case, meaning “in, inside”

So:

rappukäytävä = stairwell
rappukäytävässä = in the stairwell

Compare:

talossa = in the house
metsässä = in the forest
rappukäytävässä = in the stairwell

What does ettei mean, and why is it one word instead of että ei?

ettei is essentially että ei (“that not / so that … not”) fused into one word. It introduces a subordinate clause of purpose or result with a negative meaning:

…, että vahinko sattuu. = …so that an accident happens.
…, ettei vahinko satu. = …so that no accident happens / so that an accident does not happen.

In modern standard Finnish, ettei is normally written as one word. It corresponds roughly to:

so that … not
in order that … not
lest …

So the second clause is:

ettei = so that not
vahinko = accident
satu = (does) happen (negative form; see next question)

Why is it vahinko satu and not vahinko sattuu?

The verb is sattua = to happen, to occur; to hurt.

Positive 3rd person singular present: vahinko sattuu = an accident happens.
Negative present: vahinko ei satu = an accident does not happen.

In Finnish, with the negative (ei), the main verb goes into a special short “connegative” form, without personal ending and usually in a weaker grade:

hän tulee → hän ei tule
se pysyy → se ei pysy
vahinko sattuu → vahinko ei satu

In ettei vahinko satu, the ei is “hidden inside” ettei (että + ei), so the following verb also appears in its negative connegative form satu, not sattuu.

It can look confusing because satu also means fairy tale, but here it’s the verb form of sattua in a negative clause.

Could I say ettei satu vahinko instead of ettei vahinko satu?

You can say ettei satu vahinko, and it is grammatical, but the neutral default order is usually:

ettei vahinko satu = literally “that accident not-happens”

The subject (vahinko) normally comes before the verb. Changing the order:

ettei satu vahinko

puts extra emphasis on the verb (“that no accident will occur”), or can sound a bit more stylistic/poetic in some contexts. In everyday neutral speech and writing, ettei vahinko satu is the most natural choice.

Why is there a comma before ettei?

Because ettei vahinko satu is a subordinate clause (a dependent clause) explaining the purpose or reason for the main clause:

Main clause: Sinun ei pidä juosta rappukäytävässä
Subordinate purpose clause: ettei vahinko satu

In written Finnish, subordinate clauses introduced by että / jotta / koska / kun / vaikka / ettei etc. are typically preceded by a comma:

Lähden nyt, jotta en myöhästy.
Hän jäi kotiin, koska oli sairas.
Sinun ei pidä juosta rappukäytävässä, ettei vahinko satu.

So the comma marks the boundary between the main clause and the subordinate clause.

How strong is Sinun ei pidä juosta? Is it a strict ban or just advice?

Sinun ei pidä juosta is usually understood as “You should not run” or “You are not supposed to run”. It can cover anything from quite strong advice to a rule, depending on context and tone.

Roughly speaking:

Älä juokse rappukäytävässä!
→ Direct command: Don’t run in the stairwell!
Sinun ei pidä juosta rappukäytävässä.
→ More descriptive / instructive: You shouldn’t / mustn’t run in the stairwell.
(Often used in rules, written instructions, polite admonitions.)
Sinä et saa juosta rappukäytävässä.
→ Explicit prohibition: You are not allowed to run in the stairwell.

So ei pidä is softer and more formal than a bare imperative, but can still express a clear rule.

Could I drop Sinun and just say Ei pidä juosta rappukäytävässä, ettei vahinko satu?

Yes, you can:

Ei pidä juosta rappukäytävässä, ettei vahinko satu.

This then sounds more like a general rule: One should not run in the stairwell… / People should not run in the stairwell… rather than addressing a specific “you”.

So:

Sinun ei pidä juosta… = aimed at “you” (singular).
Ei pidä juosta… or Rappukäytävässä ei pidä juosta… = general instruction or rule.

All versions are grammatical; the choice is about who is being addressed and the style.

Why does Finnish use the present tense satu when English says “will happen” (future)?

Finnish usually uses the present tense for both:

present time
near future / general future

Context tells you whether it’s about now or the future. So:

ettei vahinko satu
literally: “so that an accident does not happen”
naturally translated: “so that no accident will happen”

Other examples:

Huomenna sataa. = It will rain tomorrow.
Tulen myöhemmin. = I will come later.

There is no separate future tense in Finnish; the simple present covers it.