Learn Hardcore Finnish: Siskoni vaihtoi taustakuvan kuvaan järvestä, ja se ilahdutti häntä koko viikon. - My sister changed the wallpaper to a picture of a lake, and it delighted her the whole week.

With a possessive suffix on the noun:
- siskoni = my sister (sisko + -ni “my”)
With a possessive pronoun + noun (often also with a suffix):
- minun siskoni = my sister

In standard written Finnish, siskoni alone is perfectly normal and usually the most natural when the context is clear. Minun siskoni adds emphasis to minun (“my”), as if contrasting: my sister (not someone else’s).

So siskoni vaihtoi… simply means my sister changed…, and there is no need to add minun unless you want that extra emphasis or contrast.

Is siskoni the subject here, and can it also mean my sister’s?

In this sentence, siskoni is the subject: my sister (nominative case).

The form siskoni is ambiguous in isolation: it can be

nominative: siskoni = my sister (subject)
genitive: siskoni = my sister’s (possessor)

You know which one it is from context and word order:

Siskoni vaihtoi… → here it is before the verb, so it’s the subject: my sister.
Siskoni taustakuva… could mean my sister’s background picture (possessive).

Why is taustakuvan in that -n form instead of just taustakuva?

Taustakuvan is the total object of the verb vaihtoi in the past tense.

Base form: taustakuva = background image
Genitive/accusative singular: taustakuvan

In Finnish, a completed action affecting the whole object often uses the genitive/accusative (-n) form as the object:

Siskoni vaihtoi taustakuvan. = She changed the (whole) background image.

Here, the same object continues into the longer structure:

Siskoni vaihtoi taustakuvan kuvaan järvestä.
= My sister changed the background image into a picture of a lake.

So taustakuvan is the total object of vaihtoi, not a possessor.

Why do we get taustakuvan kuvaan – isn’t repeating kuva (image / picture) strange?

There are actually two different kuva-words here:

taustakuva = background image (a compound noun: tausta + kuva)
kuvaan = into a picture (illative case: kuva + -an)

The structure with vaihtaa is:

vaihtaa X Y:hyn = to change/switch X into Y

So literally:

vaihtoi taustakuvan kuvaan järvestä
= changed the background image into a picture from a lake

Yes, it sounds a bit repetitive, and more idiomatic alternatives exist, e.g.:

Siskoni vaihtoi taustakuvansa järvikuvaksi.
Siskoni vaihtoi taustakuvan järvimaisemaan.

But grammatically, taustakuvan kuvaan is fine: one is “background image”, the other is “into a (new) picture”.

What is the function of kuvaan and why is it in that -an form?

Kuvaan is the illative form of kuva (“picture”):

kuva → kuvaan = into a picture

In Finnish, the verb vaihtaa often uses an illative to show what something is changed into:

vaihtaa vaatteet juhla-asuiksi = change clothes into party clothes
vaihtaa kanavan uutisiin = change the channel to the news
vaihtaa taustakuvan kuvaan järvestä = change the background image into a picture of a lake

So kuvaan here expresses the new state/result: “into a picture”.

Why is järvestä used? How does järvestä mean of a lake?

Järvestä is the elative case of järvi (“lake”):

järvi → järvestä = from a lake

With kuva (“picture”), Finnish often uses the elative to express what the picture is of:

kuva koirasta = a picture of a dog
kuva perheestä = a picture of the family
kuva järvestä = a picture of a lake

Literally it’s “a picture from a lake”, but idiomatically it corresponds to English “a picture of a lake”.

So kuvaan järvestä = into a picture of a lake.

What does se refer to in ja se ilahdutti häntä if Finnish has hän for “he/she”?

In standard Finnish:

hän = he / she (human, personal pronoun)
se = it (thing or animal; in colloquial speech, also people, but that’s informal)

In this sentence,

se = it refers to the new background picture (the picture of the lake)
häntä = her/him refers to the sister.

So:

…ja se ilahdutti häntä…
= …and it pleased / delighted her…

The difference between se and hän is about referring to things vs. people. Here, se correctly refers to a thing (the picture).

Why is it häntä and not hänet after ilahdutti?

Häntä is the partitive form of hän:

hän (nominative)
häntä (partitive)
hänet (accusative)

Many verbs of emotion, feeling, or psychological impact take their object in the partitive, especially when the effect is ongoing, not bounded, or abstract:

Se ilahdutti häntä. = It pleased/delighted her.
Se suretti häntä. = It saddened her.
Se huolestutti häntä. = It worried her.

Using hänet would sound odd here, almost like “it delighted her (completely as a whole object)”, which doesn’t match normal usage.

So ilahduttaa + partitive (häntä) is the natural pattern.

Why is koko viikon with viikon (genitive/accusative), not viikkoa?

Viikon here is the accusative (total object) form of viikko used as an expression of duration:

koko viikon = (for) the whole week

The word koko tends to go with a complete, delimited amount, so the time word is in the genitive/accusative form:

koko päivän = the whole day
koko vuoden = the whole year
koko viikon = the whole week

You could say viikkoa in other contexts (e.g. muutaman viikon / viikkoa), but with koko and a clearly bounded time span, koko viikon is the normal phrasing.

So se ilahdutti häntä koko viikon means it delighted her for the entire week, a complete period.

Is ilahdutti just the past tense of ilahduttaa, and how does it differ from iloitsi?

Yes:

ilahduttaa (verb) = to make someone happy / to delight someone
ilahdutti = past 3rd person singular: (it) delighted / pleased

Compare:

Siskoni iloitsi uudesta taustakuvasta.
= My sister was glad / rejoiced about the new background image.
(iloita = to be glad; the person themselves is happy.)
Uusi taustakuva ilahdutti siskoani.
= The new background image delighted my sister.
(ilahduttaa = to cause joy to someone; the picture is the cause.)

In your sentence, se ilahdutti häntä focuses on the picture as the cause of her happiness, not just the fact that she was happy.

Why is there a comma before ja in …, ja se ilahdutti häntä koko viikon?

In Finnish, you put a comma before ja when it connects two independent clauses, each with its own subject and verb:

Siskoni vaihtoi taustakuvan kuvaan järvestä,
(clause 1: subject = siskoni, verb = vaihtoi)
ja se ilahdutti häntä koko viikon.
(clause 2: subject = se, verb = ilahdutti)

Because both sides could be full sentences on their own, the comma is required in standard written Finnish:

Siskoni vaihtoi taustakuvan kuvaan järvestä.
Se ilahdutti häntä koko viikon.

When ja connects just two words or short phrases (not full clauses), you usually don’t add a comma, e.g. siskoni ja veljeni (“my sister and my brother”).

Could the word order be …ja häntä se ilahdutti koko viikon? Would that change the meaning?

Yes, …ja häntä se ilahdutti koko viikon is grammatically correct. Finnish word order is relatively flexible, and moving elements often affects emphasis, not core meaning.

ja se ilahdutti häntä koko viikon
= neutral: and it delighted her the whole week.
ja häntä se ilahdutti koko viikon
≈ and it was *her that it delighted the whole week
(slight emphasis on *häntä, as opposed to someone else).

So the main meaning stays the same, but the focus shifts a bit to häntä.