Learn Hardcore Finnish: Ystäväni on kiitollinen vanhemmilleen, koska he tukevat hänen opintojaan. - My friend is grateful to her parents because they support her studies.

Here minun is the separate pronoun my, and -ni repeats the possession. Both ystäväni and minun ystäväni are correct; using just ystäväni is slightly more compact and very common.

Why is the verb on used here, not olen?

The verb olla (to be) conjugates like this in the present tense:

minä olen – I am
sinä olet – you (sg) are
hän on – he/she is
me olemme – we are
te olette – you (pl) are
he ovat – they are

The subject of the sentence is Ystäväni = my friend. That is a 3rd person singular subject (like hän), so the verb must also be 3rd person singular:

Ystäväni on kiitollinen… = My friend is grateful…

Using olen would mean I am grateful…, which would not match the subject Ystäväni.

Why is kiitollinen in this form? Should it change for number or case?

Kiitollinen is an adjective meaning grateful.

In this sentence it is a predicative adjective linked to the subject by olla:

Ystäväni on kiitollinen = My friend is grateful.

For a singular subject, the predicative is in the nominative singular:

Ystäväni on kiitollinen.

If the subject were plural, the adjective would also be plural partitive:

He ovat kiitollisia. = They are grateful.

So here kiitollinen is singular because Ystäväni is one person.

What does vanhemmilleen mean, and how is it built?

Vanhemmilleen roughly means to his/her parents (or to their parents depending on context). It is built like this:

vanhempi = parent (also “older” as an adjective)
plural nominative: vanhemmat = parents
allative plural stem: vanhemmille = to the parents
- -lle is the allative ending: direction towards a person/thing, often “to” someone.
possessive suffix -en = his/her/their

So:

vanhemmille = to the parents
vanhemmilleen = to his/her parents (or their parents)

The verb/adjective combination here is:

olla kiitollinen jollekin = to be grateful to someone

That’s why vanhemmilleen is in the allative (-lle) case.

Why is there a comma before koska, and what does koska do?

Koska is a conjunction meaning because, introducing a reason clause:

…, koska he tukevat hänen opintojaan.
= …, because they support her studies.

In standard written Finnish, a comma is normally used before a subordinate clause introduced by words like koska, että, vaikka etc.

You can also move the koska-clause to the beginning:

Koska he tukevat hänen opintojaan, ystäväni on kiitollinen vanhemmilleen.

The comma stays, but now it comes after the subordinate clause.

Who does he refer to in koska he tukevat hänen opintojaan?

He is the 3rd person plural pronoun = they.

It refers back to vanhemmilleen (his/her parents). So:

vanhemmilleen → his/her parents
he tukevat → they support

So the second clause means: because *they (the parents) support her studies*.

How is tukevat related to the dictionary form tukea, and why is it in this form?

The dictionary form tukea is the infinitive “to support”.

The present tense forms of tukea are:

minä tuen – I support
sinä tuet – you (sg) support
hän tukee – he/she supports
me tuemme – we support
te tuette – you (pl) support
he tukevat – they support

Because the subject is he (they), we use tukevat, the 3rd person plural present tense form:

he tukevat = they support

Why is it hänen opintojaan when we already have Ystäväni (= my friend)? Who does hänen refer to?

Hänen is the 3rd person singular possessive pronoun = his/her.

Although Ystäväni contains “my”, it refers to the friend as a 3rd person (he/she), not as me. The sentence means:

My friend is grateful to her parents because they support *her studies.*

So:

Ystäväni = my friend
hänen opintojaan = her studies (belonging to that friend)

If you said minun opintojani, it would mean my studies (the speaker’s), which would change the meaning:

Ystäväni on kiitollinen vanhemmilleen, koska he tukevat minun opintojani.
= My friend is grateful to her parents because they support *my studies.*

What exactly is going on in the word opintojaan?

Opintojaan can be broken down like this:

opinto = a study (as in university studies)
plural partitive: opintoja = studies (some studies, her studies in general)
- -ja here marks plural partitive.
possessive suffix: opintoja
- -n → opintojaan

The -n at the end is the 3rd person possessive suffix, showing that the studies belong to him/her:

opintoja = studies
opintojaan = his/her studies

So hänen opintojaan means her studies (in the partitive plural).

Why is opintojaan in the partitive case, not nominative (like opinnot)?

The verb tukea (to support) often takes the partitive object when the action is:

ongoing / not completed, or
affecting something in an open, not-bounded way, or
talking about something abstract or uncountable in general.

Compare:

he tukevat hänen opintojaan = they support her studies (in general, ongoing)
he tukevat hänen opintonsa – would sound like supporting a specific, more delimited set of studies; not the usual way to say this.

So opintojaan is:

partitive plural = opintoja
plus possessive suffix -n = his/her

This fits the natural Finnish expression for supporting (someone’s) studies as an ongoing process.

Can the word order be changed in this sentence? For example, can the koska-clause come first?

Yes, Finnish allows flexible word order for emphasis.

Some natural variants:

Original:
- Ystäväni on kiitollinen vanhemmilleen, koska he tukevat hänen opintojaan.
Koska-clause first (emphasizing the reason):
- Koska he tukevat hänen opintojaan, ystäväni on kiitollinen vanhemmilleen.
Moving vanhemmilleen:
- Ystäväni on vanhemmilleen kiitollinen, koska he tukevat hänen opintojaan.
  (more emphasis on to her parents)

All of these are grammatically correct; the differences are mainly in focus and emphasis, not in basic meaning.

Could we say Ystäväni on kiitollinen vanhemmistaan instead of vanhemmilleen? What is the difference?

Both are possible, but they express slightly different ideas:

kiitollinen jostakin (+ -sta/-stä, elative)
- Ystäväni on kiitollinen vanhemmistaan.
  = My friend is grateful *for her parents.
  Here the parents themselves are the *thing she is grateful for.
kiitollinen jollekin (+ -lle, allative)
- Ystäväni on kiitollinen vanhemmilleen.
  = My friend is grateful *to her parents.
  Here the parents are the *people to whom she feels gratitude (typically for what they have done).

In your original sentence, vanhemmilleen fits very well, because the next clause explains what they do:
they support her studies, and she is grateful to them for that.