Kun hiiri ei toimi, en voi avata yhtään tärkeää tiedostoa.

Both kun and jos can sometimes be translated as when or if, but they are not the same.

• kun = when, whenever, as
  Used for:

  • time relations (when something happens)
  • things the speaker treats as real or expected

• jos = if
  Used for:

  • conditions that are more hypothetical or uncertain
  • typical “if… then…” structures

In this sentence:

• Kun hiiri ei toimi, en voi avata…
  = When / whenever the mouse doesn’t work, I can’t open…

It presents the mouse-not-working as a real, recurring situation.

You could say:

• Jos hiiri ei toimi, en voi avata…
  = If the mouse doesn’t work, I can’t open…

This sounds a bit more conditional, like a rule: If that situation happens, then this follows. Both are grammatically correct; kun is just a bit more factual / temporal here.

Finnish uses a comma between a subordinate clause and the main clause, especially when the subordinate clause comes first.

• Subordinate clause: Kun hiiri ei toimi,
• Main clause: en voi avata yhtään tärkeää tiedostoa.

The rule is:
If a sentence starts with a conjunction like kun, koska, vaikka, jos introducing a clause, you normally put a comma before the following main clause.

So the comma is mandatory here:
Kun hiiri ei toimi, en voi avata…

hiiri literally means mouse and can be:

• a real mouse (the animal),
• a computer mouse (the device).

Here, context tells us it’s the computer mouse:

• hiiri ei toimi = the mouse doesn’t work / isn’t functioning
• tiedostoa = file

Talking about files and something not working strongly suggests a computer context, so a computer mouse.

This is about the Finnish negative verb.

In Finnish, negation is expressed with a special verb ei, which carries the person/number, while the main verb goes into a special “connegative” form (no personal ending).

For toimia (to function, to work):

• Affirmative:
  • se toimii = it works
• Negative:
  • se ei toimi = it does not work

Notice:

• Affirmative 3rd person: toimi-i → toimii
• Negative 3rd person: ei toimi (no -i ending)

So ei toimii is wrong; the correct form is ei toimi.

Again, this is the negative verb.

The verb voida (can, be able to) in the present tense:

• Affirmative:

  • minä voin = I can
  • sinä voit = you can
  • hän voi = he/she can

• Negative:

  • minä en voi = I cannot
  • sinä et voi = you cannot
  • hän ei voi = he/she cannot

The negative verb (en, et, ei, emme, ette, eivät) carries the person, so voida loses its personal ending:

• not en voin
• but en voi

So en voi avata = I can’t open.

avata is in the infinitive (the “dictionary form”) because it comes after a modal verb, voida (en voi avata).

In Finnish:

• After voida (can), haluta (want to), täytyä (must), etc., the following verb stays in the 1st infinitive:
  • voi avata = can open
  • haluan avata = I want to open
  • täytyy avata = (it) must be opened / one must open

So:

• En voi avata = I cannot open
  (not en voi avaan or en voin avata)

yhtään in this context means any (at all) and intensifies the negation.

• en voi avata yhtään tärkeää tiedostoa
  ≈ I can’t open any important files at all.

Typical usage:

• Mostly in negative or restrictive contexts:
  • en ymmärrä yhtään = I don’t understand at all
  • ei ole yhtään rahaa = there isn’t any money

It sounds odd or at least very marked in a clearly positive sentence. So it naturally appears with ei, älä, etc., to stress the complete absence of something.

Both tärkeää and tiedostoa are in the partitive singular, and they agree with each other.

• Base forms (nominative singular):

  • tärkeä = important
  • tiedosto = file

• Partitive singular:

  • tärkeää
  • tiedostoa

Reasons:

1. Negative object → partitive
   In Finnish, the direct object in a negative sentence is normally in the partitive:

   • Avaan tiedoston. = I open the file. (affirmative, total object, nominative)
   • En avaa tiedostoa. = I don’t open the file. (negative, partitive)

2. Indefinite / “any” meaning
   With yhtään and a negative, the partitive also reflects the idea of any amount / not even one.

3. Adjective–noun agreement
   Adjectives agree in case and number with the noun:

   • tärkeä tiedosto (nom. sg.)
   • tärkeää tiedostoa (part. sg.)

So yhtään tärkeää tiedostoa is structurally:
yhtään (any) + tärkeää (important, partitive) + tiedostoa (file, partitive).

tiedostoa is partitive singular.

Formation from tiedosto:

• tiedosto (nominative singular)
• Add -a or -ä according to vowel harmony; because of o, we use -a:
  • tiedosto + a → tiedostoa

So:

• tiedosto = file (subject / dictionary form)
• tiedostoa = file (as a partitive object: some file, any file, or object in a negative sentence)

Yes, you can say:

• En voi avata tärkeää tiedostoa.

Grammatically, this is fine. The difference is in meaning:

• En voi avata yhtään tärkeää tiedostoa.
  = I can’t open any important file(s) at all.
  → clearly indefinite, “zero of them”.

• En voi avata tärkeää tiedostoa.
  More naturally interpreted as:

  • I can’t open the important file
    (some particular important file that we both know about),
    or at least less strongly “any at all”.

So yhtään emphasizes none at all / not a single one and pushes the interpretation towards any important file(s) rather than a specific one.

Grammatically, it is singular partitive, but its meaning can be equivalent to English any important file(s).

In Finnish, the singular partitive is often used for:

• indefinite, unspecified items,
• “any X” in negative sentences,
• quantities that in English would be expressed with a plural.

So:

• yhtään tärkeää tiedostoa
  literally: any important file (part.)
  but often best translated as: any important files.

If you really want a clear plural in Finnish, you could say:

• yhtään tärkeää tiedostoa (very natural, default)
• tärkeitä tiedostoja (partitive plural), but you would normally combine that with mitään or similar in a carefully phrased sentence.

Yes, you can, with slightly different nuance:

• mitään tärkeää tiedostoa

  • mitään = anything / any … at all
  • Sounds a bit more general, very common with negation:
    • En voi avata mitään tärkeää tiedostoa.
      ≈ I can’t open any important file at all.

• yhtäkään tärkeää tiedostoa

  • yhtäkään ≈ not even a single / not one
  • Stronger emphasis on not even one:
    • En voi avata yhtäkään tärkeää tiedostoa.
      ≈ I can’t open a single important file.

• yhtään tärkeää tiedostoa

  • Very common neutral choice meaning any … at all, slightly less dramatic than yhtäkään, more specific than mitään alone.

All three are grammatically fine; it’s mainly about emphasis and style.

Yes, that is perfectly grammatical:

• En voi avata yhtään tärkeää tiedostoa, kun hiiri ei toimi.

Finnish allows either order:

1. Kun hiiri ei toimi, en voi avata yhtään tärkeää tiedostoa.
2. En voi avata yhtään tärkeää tiedostoa, kun hiiri ei toimi.

The factual meaning is the same. The difference is a small shift in emphasis:

• Version 1 starts from the condition (when the mouse doesn’t work) and then states the consequence.
• Version 2 starts from your inability (I can’t open any important files) and then gives the reason (because the mouse doesn’t work).

In spoken language, the second version often has more of a causal feel, almost like because.

No, not in normal, neutral Finnish.

Adjectives like tärkeä almost always come before the noun they modify:

• tärkeä tiedosto, uusi auto, iso talo

In most standard contexts, the adjective cannot simply be moved after the noun like in some languages:

• ✗ yhtään tiedostoa tärkeää (ungrammatical in neutral Finnish)

You can sometimes put an adjective after the noun for very marked, poetic, or stylistic reasons, but that’s not the normal structure and not something to imitate as a learner. So here you must say:

• yhtään tärkeää tiedostoa

You need to put both verbs into the past tense, including the negative structure:

• Kun hiiri ei toiminut, en voinut avata yhtään tärkeää tiedostoa.

Breakdown:

• hiiri ei toiminut

  • toimia (to work/function)
  • toimi (past stem) + -nut participle → toiminut
  • Negative past: ei toiminut = did not work

• en voinut avata

  • voida (can) → voinut in past negative
  • en voinut = I could not / was not able to
  • avata stays in the infinitive after voida

So:

• Present: Kun hiiri ei toimi, en voi avata…
• Past: Kun hiiri ei toiminut, en voinut avata…