Learn Hardcore Finnish: Kännykkä sammuu, jos akku on tyhjä. - The mobile phone turns off if the battery is empty.

sammuu is the 3rd person singular present of sammua = to go out, to switch off by itself (intransitive).
sammuttaa = to turn something off (transitive; needs an agent). Examples:
Kännykkä sammuu. The phone shuts down (by itself).
Sammutan kännykän. I turn the phone off.

Why does Finnish use the present tense here when English would say “will shut down”?

Finnish has no separate future tense. The simple present covers both present and future. In if-clauses you normally use the present in both clauses: Jos akku on tyhjä, kännykkä sammuu.

Is the comma before jos required?

Yes. Finnish uses a comma before most subordinate clauses, including jos-clauses, even when the subordinate clause comes last: Kännykkä sammuu, jos akku on tyhjä.

Can I put the jos-clause first?

Yes: Jos akku on tyhjä, kännykkä sammuu. Keep the comma after the jos-clause. You can optionally add niin: Jos akku on tyhjä, niin kännykkä sammuu.

What’s the nuance difference between tyhjä, loppu, tyhjenee, and loppuu with batteries?

Akku on tyhjä. The battery is empty (state).
Akku on loppu. The battery is dead/out (very common).
Akku tyhjenee. The battery is emptying (process).
Akku loppuu. The battery is running out / about to die (process), or in past Akku loppui.

Why is it tyhjä and not the partitive tyhjää?

With a countable subject in the nominative (akku), the predicative adjective is nominative: Akku on tyhjä. Partitive predicatives (e.g., Vesi on kylmää) are used with mass nouns/indefinite amounts, not with a single countable akku.

Why is it akku and not akun?

Akku is the subject, so it’s nominative. Akun (genitive) appears in other structures, e.g., akun ollessa tyhjä (“when the battery is empty”), but not as a simple subject of on.

Is kännykkä formal? Could I use puhelin?

kännykkä is widely used and fine in most contexts; slightly colloquial.
puhelin is neutral “phone.”
More formal/technical: matkapuhelin (mobile phone), älypuhelin (smartphone). Your sentence also works as Puhelin sammuu, jos akku on tyhjä.

What’s the difference between akku and paristo?

akku = rechargeable battery (phones, laptops, cars).
paristo = non-rechargeable battery (AA/AAA, coin cells).

Could I say jos akku tyhjenee instead of jos akku on tyhjä?

Yes, but it changes the focus:

jos akku on tyhjä = if the battery is already empty (state).
jos akku tyhjenee = if the battery empties (event/process).

What’s the difference between jos and kun here?

jos = if (conditional, hypothetical).
kun = when (temporal, expected/factual). For a general law: Kännykkä sammuu, kun akku on tyhjä. This asserts it happens whenever that condition occurs.

Can I drop on and say jos akku tyhjä?

No. Finnish generally requires olla in such sentences. Say jos akku on tyhjä.

Is tyhjänä possible instead of tyhjä?

Sometimes, but here the nominative predicate is standard: Akku on tyhjä. The essive tyhjänä (“in an empty state”) is common with containers (e.g., Pullo on tyhjänä) but is less typical for batteries.

How do I say “unless the battery is empty, the phone won’t shut down”?

Use ellei and the negative verb:

Ellei akku ole tyhjä, kännykkä ei sammu.

Does kännykkä sammuu refer to one specific phone or phones in general?

Finnish has no articles. A bare singular often expresses a general truth. Kännykkä sammuu, jos akku on tyhjä ≈ “A/your/the phone shuts down when its battery is empty,” i.e., phones in general. Context decides specificity.

How do I pronounce the tricky parts: kännykkä sammuu?

ä = a as in “cat.”
y = French u / German ü.
Double consonants (nn, kk, mm) and double vowels (uu) are long; hold them. Roughly: KÄNN-ykkä SAMM-uu.

Why not sammuu pois or sulkeutuu?

sammuu alone already means “shuts down/goes off.” pois is usually unnecessary.
sulkeutua = to close/shut (intr.). For a device losing power, sammua is the most idiomatic verb.

Would Finnish ever use a conditional like sammuisi in this kind of sentence?

Real/general condition: present in both clauses — Jos akku on tyhjä, kännykkä sammuu.
Hypothetical/unreal: conditional in both — Jos akku olisi tyhjä, kännykkä sammuisi.