Learn Hardcore Finnish: Minun lupaukseni on, että opiskelen suomea joka päivä, ja yritän todella toteuttaa sen. - My promise is that I study Finnish every day, and I really try to fulfil it.

So yes, the meaning “my” is marked twice, but this is very normal and stylistically quite neutral in modern Finnish, especially with a pause or emphasis on minun.

You could also say:

Lupaukseni on, että… – perfectly correct and a bit more compact.
Minun lupaus on, että… – colloquial, grammar-wise a bit “looser”; you’ll hear this in speech.

In careful written Finnish, minun lupaukseni or just lupaukseni is preferred.

What case is lupaukseni in, and how is it formed from lupaus?

Lupaukseni is:

in the nominative singular (basic subject form), and
has the 1st person singular possessive suffix -ni (my).

Formation:

Basic word: lupaus (promise)
Stem: lupaukse- (you see this in many inflected forms)
Nominative with possessive: lupaukseni
- the -se- part disappears in this form: lupaukse- → lupauks-
- plus -ni → lupaukseni

So the full breakdown is: lupaus (promise) + -kse- (stem element) + -ni (my).

Why is the verb on used here: Minun lupaukseni on, että…? Could you say it differently?

On is the 3rd person singular form of olla (to be). The pattern is:

X on, että Y = X is that Y…

So the structure is:

Minun lupaukseni on, että opiskelen…
= My promise is that I study…

Common alternatives:

Lupaukseni on opiskella suomea joka päivä.
(using an infinitive instead of an että-clause)
Olen luvannut opiskella suomea joka päivä.
(I have promised to study Finnish every day.)

The original is very natural: [Subject] + on, että + [clause].

What does että do in että opiskelen suomea joka päivä?

Että is a conjunction meaning “that” (introducing a subordinate clause), similar to English:

My promise is *that I will study Finnish every day.*

So:

Main clause: Minun lupaukseni on
Subordinate että-clause: että opiskelen suomea joka päivä

The comma before että is required in written Finnish when you introduce this kind of että-clause.

Why is opiskelen present tense even though in English we might say “I will study Finnish every day”?

Finnish often uses the present tense where English uses the future.

opiskelen suomea joka päivä
literally: I study / I am studying Finnish every day
but in context: I will study Finnish every day (ongoing/future habit)

There’s no separate future tense in Finnish. Future meaning is usually expressed by:

present tense + a time expression:
opiskelen suomea joka päivä – clearly about a repeated action over time
or by context alone.

So using the present here is fully correct and normal.

Why is it suomea and not suomi in opiskelen suomea?

Suomea is the partitive singular form of suomi (Finnish).

The verb opiskella (to study, to learn by studying) typically takes its object in the partitive, especially when you are talking about:

studying / learning a subject in general (an ongoing, not-fully-delimited process)

Examples:

opiskelen suomea – I study Finnish
opiskelen historiaa – I study history
opiskelen matematiikkaa – I study mathematics

So suomea is the usual and idiomatic choice after opiskelen.

What does joka päivä mean exactly, and can it be placed elsewhere in the sentence?

Joka päivä literally means “every day”.

joka = every / each
päivä = day

It functions as a time adverbial and is quite flexible in word order:

Opiskelen suomea joka päivä.
Joka päivä opiskelen suomea. (slightly more emphasis on every day)
In the full sentence:
- …että opiskelen suomea joka päivä (neutral)
- …että joka päivä opiskelen suomea (emphasis on every day)

All are grammatically correct; the difference is mostly nuance and emphasis.

What does yritän mean, and why is it in the first person singular form?

Yritän is the 1st person singular present tense of the verb yrittää:

yrittää = to try, to attempt
yritän = I try / I will try

In the sentence:

…ja yritän todella toteuttaa sen.
= …and I really try to carry it out.

Since the speaker is the one making and keeping the promise, yritän (I try) is the natural choice.

What is the role of todella in yritän todella toteuttaa sen? Could it go somewhere else?

Todella here is an adverb meaning roughly “really / truly / genuinely”, reinforcing how seriously the person is trying.

Word order options:

yritän todella toteuttaa sen (very natural, adverb after the verb)
todella yritän toteuttaa sen (emphasizes the “really”)
yritän toteuttaa sen todella (can sound like carry it out in a real way, more context-dependent)

The most neutral for “I really try to carry it out” is:

yritän todella toteuttaa sen

What does toteuttaa mean here, and how is it different from verbs like pitää or täyttää a promise?

Toteuttaa means “to carry out, to realize, to put into practice”.

toteuttaa lupaus = to carry out / fulfill a promise (implement it in real life)

Other verbs you might see with lupaus (promise):

pitää lupaus = literally to keep a promise (very common, idiomatic)
täyttää lupaus = to fulfill a promise (more formal-sounding)

So:

yritän todella toteuttaa sen = I really try to carry it out / make it happen
You could also say: yritän todella pitää sen or yritän todella pitää lupaukseni
(meaning I really try to keep it / keep my promise).

What does sen refer to in yritän todella toteuttaa sen, and why is it sen and not sitä?

Sen is a pronoun referring back to minun lupaukseni (my promise).

lupaus is a t-word noun: se / sen / sitä, etc.
Here we have sen, which can function as the genitive or accusative form.

Why sen and not sitä?

With a full, definite object that you are aiming to “complete” (carry out fully), Finnish tends to use the total object (genitive/accusative-like form): sen.
Sitä would be the partitive form and would suggest something more open-ended or incomplete, which doesn’t fit as well with the idea of fully carrying out a promise.

So:

toteuttaa sen ≈ carry it out (completely)
sen = that (promise), referring to minun lupaukseni.

Why does the sentence use ja (and) before yritän, and can the second clause stand alone?

The sentence:

Minun lupaukseni on, että opiskelen suomea joka päivä, ja yritän todella toteuttaa sen.

has two coordinated parts connected by ja (and):

Minun lupaukseni on, että opiskelen suomea joka päivä
(minä) yritän todella toteuttaa sen

The second part could stand alone as:

Yritän todella toteuttaa sen.
= I really try to carry it out.

Using ja links the definition of the promise with the effort to keep it, showing:

not only what the promise is,
but also that the speaker is actively trying to fulfill it.

So ja is natural and adds that extra nuance of “and also”.

Could you rephrase the sentence in a slightly more compact but still natural way?

Yes, for example:

Lupaukseni on opiskella suomea joka päivä, ja yritän todella pitää sen.
(My promise is to study Finnish every day, and I really try to keep it.)

Or:

Olen luvannut opiskella suomea joka päivä ja yritän todella toteuttaa sen.
(I have promised to study Finnish every day, and I really try to carry it out.)

The original sentence is already natural; these are just stylistic variations.