Learn Hardcore Finnish: Kirjoitan illalla lyijykynällä pieneen harjoituspäiväkirjaan, mitä olen oppinut tänään. - In the evening I write with a pencil in a small practice diary what I have learned today.

The part mitä olen oppinut tänään functions as the object of kirjoitan (“I write what I have learned today”), and all such subordinate clauses are separated with a comma in standard written Finnish.

What exactly is mitä doing here? Why not että?

Mitä here is an interrogative/relative pronoun meaning what. It introduces a content clause that tells what you write:

kirjoitan … mitä olen oppinut tänään
= I write what I have learned today.

If you used että, the meaning would change:

Kirjoitan, että olen oppinut tänään paljon.
= I write that I have learned a lot today.

So:

että = that (introduces a statement clause)
mitä = what (introduces a clause about the content of “what”)

Why is it mitä and not mikä?

Mikä and mitä are forms of the same word:

mikä = nominative (subject or basic form)
mitä = partitive singular

Here, mitä is the object of olen oppinut (“what I have learned”). The partitive is used because:

The “what” here is indefinite / unspecified (basically “what things I have learned”), and Finnish often uses the partitive for this kind of “some amount of something”.
With verbs like oppia (“to learn”), a partitive object is common when talking about learning “some things”, not a single clearly delimited item.

So mitä ≈ “what (things)” in this context, and that calls for the partitive form.

Why is kirjoitan in the present tense if it refers to the future (“this evening”)?

Finnish uses the present tense much more broadly than English. It can cover:

present time
near future, when a time expression makes that clear

Here, illalla (“in the evening”) tells us it’s about a future time today, so kirjoitan naturally means:

Kirjoitan illalla… = I will write / I’m going to write this evening.

Using a separate future tense is not needed; Finnish doesn’t have one.

What does illalla mean grammatically, and why that form instead of a preposition like “in the evening”?

Illalla is the adessive case of ilta (evening):

ilta = evening (basic form)
illalla = in the evening / at evening / this evening

Finnish usually doesn’t use prepositions like “in” or “at” for times; instead, it marks time expressions with cases. Some common ones:

illalla – in the evening (once / today)
iltaisin – in the evenings (habitually)
yöllä – at night
huomenna – tomorrow (adverb, not case, but same idea: no preposition)

So illalla itself already includes the idea of “in the evening”; you don’t add a separate preposition.

Why is lyijykynällä in that form? How does Finnish say “with a pencil”?

Lyijykynällä is also in the adessive case, and here the adessive expresses the instrument:

lyijykynä = pencil
lyijykynällä = with a pencil / by pencil

Finnish often uses the adessive case for tools and instruments:

kirjoitan kynällä – I write with a pen
leikkaan veitsellä – I cut with a knife
maksoin kortilla – I paid with a card

So instead of a preposition “with”, Finnish modifies the noun with the adessive ending -lla/-llä.

What case is pieneen harjoituspäiväkirjaan, and why do both words change?

Pieneen harjoituspäiväkirjaan is in the illative case, which usually corresponds to “into / to / in(to)” some place or container.

Breakdown:

pieni (small)
→ pieneen (into a small …)
harjoituspäiväkirja (practice diary)
→ harjoituspäiväkirjaan (into a practice diary)

In Finnish, an adjective must agree in case and number with the noun it modifies:

nominative: pieni harjoituspäiväkirja – a small practice diary
illative: pieneen harjoituspäiväkirjaan – into a small practice diary

You use the illative here because kirjoittaa normally takes an illative object when you mean “write into (something)”:

kirjoitan päiväkirjaan – I write in / into the diary
kirjoitan vihkoon – I write into the notebook

How does the compound harjoituspäiväkirja work? Could it be three separate words?

Harjoituspäiväkirja is a compound noun made of:

harjoitus – exercise / practice
päiväkirja – diary (literally “day-book”)

So harjoituspäiväkirja = a diary used for exercises/practice.

You cannot write it as three separate words (harjoitus päivä kirja) in normal Finnish; that would break the meaning. The basic structure is:

harjoitus + päiväkirja → harjoituspäiväkirja
(then you add case endings: harjoituspäiväkirjaan in this sentence)

Finnish loves compound nouns; what would be “practice diary” in English is usually a single word in Finnish.

Why use olen oppinut instead of a simple past like opin?

Olen oppinut is the perfect tense (literally “I am having-learned”), similar to English I have learned.

Nuance:

olen oppinut – focuses on the result that still matters now; it fits well with “today” and the idea of summing up what you’ve learned during the day.
opin – simple past (“I learned”); more about the event itself in the past, less about its present relevance.

In this context, the writer is recording the result of today’s learning in the diary, so the perfect (olen oppinut) is the natural choice.

Could the word order mitä olen oppinut tänään be mitä olen tänään oppinut, and would that change the meaning?

Yes, both are grammatically correct:

mitä olen oppinut tänään
mitä olen tänään oppinut

They mean essentially the same: what I have learned today.

The difference is very subtle and mostly about rhythm and focus:

mitä olen oppinut tänään – neutral, very common. “Today” comes at the end.
mitä olen tänään oppinut – puts a bit more emphasis on today by bringing it closer to the verb; some speakers might prefer this rhythm.

In everyday use, they are interchangeable here.

Could I say Kirjoitan pieneen harjoituspäiväkirjaan illalla lyijykynällä instead? How flexible is the word order?

Yes, you can. Finnish word order is quite flexible, because case endings show the grammatical roles.

All of these are possible and mean the same in neutral context:

Kirjoitan illalla lyijykynällä pieneen harjoituspäiväkirjaan…
Kirjoitan illalla pieneen harjoituspäiväkirjaan lyijykynällä…
Kirjoitan pieneen harjoituspäiväkirjaan illalla lyijykynällä…

What changes is emphasis and what feels more natural:

Items earlier in the sentence are often background or known information.
Items later (especially near the end of the clause) can get a bit more focus.

Your suggested order is completely fine; it just slightly shifts the rhythm and what the speaker might be subtly highlighting.

How would the sentence change if I wanted to say “into my small practice diary”?

You would usually mark possession with a possessive suffix:

harjoituspäiväkirja – practice diary
harjoituspäiväkirja
- -ni (my) → harjoituspäiväkirjani
illative: harjoituspäiväkirjaani – into my practice diary

The full phrase:

pieneen harjoituspäiväkirjaani = into my small practice diary

So the whole sentence could be:

Kirjoitan illalla lyijykynällä pieneen harjoituspäiväkirjaani, mitä olen oppinut tänään.

You could also add minun for extra clarity or emphasis, but it’s usually not necessary:

Kirjoitan illalla lyijykynällä minun pieneen harjoituspäiväkirjaani… (more emphatic “my”)