Learn Hardcore Finnish: Työpöydän taustakuva on nyt metsä, koska se rauhoittaa mielentilaa. - The desktop wallpaper is now a forest because it calms my state of mind.

Työpöydän is the genitive form of työpöytä (desktop). In Finnish, the genitive (X:n) is often used to show that something belongs to or is associated with something else.

So:

työpöytä = (a) desktop
työpöydän = of the desktop
taustakuva = background image / wallpaper

työpöydän taustakuva literally = the desktop’s background image → the desktop background.

This X:n Y structure is very common:

puhelimen näyttö = the phone’s screen
ikkunan verhot = the window’s curtains

Could you also say työpöytät taustakuva as one word, or make a compound noun here?

No, työpöydän taustakuva is not written as one compound word.

työpöytä itself is already a compound (työ
- pöytä).
Then we add taustakuva to it with a genitive: työpöydän taustakuva.

When the first part is already a full word and we want to show possession/association, Finnish usually uses the genitive, not another compound:

auton ovi (the car door), not ❌ auto-ovi (though some fixed compounds do exist, like takaluukku).
tietokoneen näyttö (the computer screen), not ❌ tietokonennäyttö.

So the normal, natural form here is työpöydän taustakuva.

Why is metsä in the basic form (nominative) and not metsänä or metsää?

Here metsä is a predicate noun after the verb olla (to be). With olla, if you are simply identifying what something is, you use the nominative:

Taustakuva on metsä. = The background picture is a forest.

Alternatives and why they’re different:

metsänä (essive): used for as a forest / in the role/state of a forest
- Taustakuva on metsänä. – Sounds odd; would suggest something like “the background is functioning as a forest”.
metsää (partitive): used more for “some forest / forest (as a substance),” or in certain aspectual contexts.
- Siellä näkyy metsää. = You can see (some) forest (there).
  In a simple X is Y identity sentence, nominative (metsä) is normal.

So metsä is correct because we’re saying: The desktop background is (a) forest.

Why is mielentilaa in the partitive case, not mielentila?

Mielentila = (a) state of mind / mood.
Mielentilaa is its partitive singular.

The object of rauhoittaa (to calm, to soothe) is usually in the partitive when we talk about affecting someone’s state in general, not completely changing it in a clear, bounded way.

se rauhoittaa mielentilaa ≈ it calms/soothes the state of mind (in general, to some extent).

Using the partitive here makes the action sound:

ongoing / partial / general, not a one-time, fully completed change.

If you used the accusative-like form mielentilan, it would sound more like a complete, definite result (it calms the state of mind fully), which is less natural in this everyday sentence. The partitive is more typical for psychological states being influenced.

What exactly does koska do here, and can the order of the clauses be changed?

Koska is a subordinating conjunction meaning because.

In this sentence:

Työpöydän taustakuva on nyt metsä = main clause
koska se rauhoittaa mielentilaa = subordinate clause (reason)

You can change the order:

Koska se rauhoittaa mielentilaa, työpöydän taustakuva on nyt metsä.
= Because it calms the state of mind, the desktop background is now a forest.

Word order between main and subordinate clauses is fairly flexible in Finnish. Putting the koska-clause first slightly emphasizes the reason. Putting it after is more neutral. The grammar (word forms and endings) doesn’t change.

What does se refer to, and does Finnish have gendered pronouns like he/she?

In koska se rauhoittaa mielentilaa, se = the forest (the image of the forest used as the background).

Finnish does not have grammatical gender in pronouns:

hän = he / she (for people; no gender distinction)
se = it (for things, animals) and also often used for people in spoken Finnish.

Here se is a neutral it, pointing back to the idea “a forest (as a background)” mentioned in the main clause. There’s no male/female distinction.

Could you also say Nyt työpöydän taustakuva on metsä or Työpöydän taustakuva on metsä nyt? Does the position of nyt matter?

All of these are grammatically correct, but the nuance and emphasis shift slightly:

Työpöydän taustakuva on nyt metsä (original)
– Neutral, common. Focus is on what it is now.
Nyt työpöydän taustakuva on metsä
– Emphasizes now as a contrast to some other time:
Now the background is a forest (maybe before it was something else).
Työpöydän taustakuva on metsä nyt
– Also possible; nyt at the end can feel a bit more spoken/colloquial or slightly emphatic on timing.

Finnish word order is flexible, but the default-neutral place for short time adverbs like nyt is typically early in the clause, often right after the verb, as in the original sentence.

What’s the difference between rauhoittaa and rauhoittua, and why is rauhoittaa used here?

rauhoittaa = to calm (something/someone) → transitive
- Metsä rauhoittaa mielentilaa. = The forest calms the state of mind.
rauhoittua = to calm down → intransitive (the subject calms itself)
- Mielentila rauhoittuu. = The state of mind calms down.

In the sentence, the forest (image) is doing the calming, and state of mind is the thing being calmed. So we need the transitive verb rauhoittaa:

se (the forest) = subject
rauhoittaa = calms
mielentilaa = object (what is calmed)

If you used rauhoittuu, you’d need mielentila as the subject:

Mielentila rauhoittuu, kun taustakuvana on metsä.
= The state of mind calms down when the background is a forest.

Could you say Työpöydällä taustakuva on nyt metsä instead of työpöydän taustakuva?

No, that would be odd and unidiomatic.

työpöydällä = on the desktop (adessive case, location)
työpöydän taustakuva = the desktop’s background image (genitive construction)

The idea is “desktop background”, which is a property of the desktop, not something simply “on top of” it. So Finnish uses the genitive:

työpöydän taustakuva = desktop background (literally: the desktop’s background image)

Työpöydällä taustakuva… would sound like you have a physical photo sitting on top of the desk.

Why is the verb on (is) in the present tense, even though in English we might say “has become” or “is now”?

Finnish present tense on covers several English nuances:

simple present: Se on metsä. = It is a forest.
present state that is new/changed: Se on nyt metsä. = It is now a forest / It has now become a forest.

The word nyt (“now”) already implies a change compared to before. Finnish doesn’t need a special “has become” form here; on nyt is enough to express:

previous state ≠ current state
currently: background = forest

So on nyt naturally includes the idea “is (now)” or “has now become” from context.

Is mielentila the only way to say “mood” here, or could you use other words?

Mielentila literally means state of mind and is a natural choice here, especially in a bit more neutral or formal style.

Other options, with slight differences in tone:

mieli = mind/mood (shorter, very common)
- se rauhoittaa mieltä = it calms the mind
olo = feeling, how one feels (informal, physical+mental)
- se rauhoittaa oloa = it makes you feel calmer
tunnelma = atmosphere, mood (of a place/situation)
- se rauhoittaa tunnelmaa = it calms the atmosphere.

In this exact sentence, mielentilaa sounds quite natural and slightly more “psychological” or descriptive of inner state.