Learn Hardcore Finnish: Illalla kirjoitan päiväkirjaan, mistä pettymyksestä opin eniten tänään. - In the evening I write in my diary from which disappointment I learned the most today.

So Illalla kirjoitan… = “In the evening I (will) write…”. There is no separate preposition like in; the case ending -lla carries that meaning.

Why is kirjoitan (present tense) used when the meaning is “I will write in the evening”?

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

Kirjoitan is the present tense of kirjoittaa (to write). In context with a future time expression like illalla, it naturally gets a future interpretation:

Illalla kirjoitan päiväkirjaan = “In the evening I’ll write in (my) diary.”

You could use a periphrastic future (tulen kirjoittamaan) in some contexts, but the plain present is by far the most typical and natural here.

Why is there a comma before mistä?

The comma marks the beginning of a subordinate clause.

Main clause: Illalla kirjoitan päiväkirjaan
Subordinate (relative) clause: mistä pettymyksestä opin eniten tänään

In Finnish, all subordinate clauses are separated from the main clause with a comma, regardless of whether you would put a comma there in English. So even though English might write:

In the evening I’ll write in my diary about which disappointment I learned the most today.

Finnish grammar requires the comma before mistä.

Is mistä a question word here? Why is it used in the middle of the sentence?

Mistä can be both:

a question word (from what? from where?) and
a relative word (from which, referring back to something).

In this sentence it works as a relative word introducing the subordinate clause:

mistä pettymyksestä = from which disappointment

So the structure is:

Main clause (what you’ll do): Illalla kirjoitan päiväkirjaan
Relative clause (what you’ll write about): mistä pettymyksestä opin eniten tänään = “from which disappointment I learned the most today”

You could paraphrase it a bit more explicitly (but more clumsily) as:
Illalla kirjoitan päiväkirjaan siitä, mistä pettymyksestä opin eniten tänään.
“Tonight I’ll write in my diary about that, about which disappointment I learned the most today.”

Why is pettymyksestä in the -sta form? What does that case mean?

Pettymyksestä is the elative case of pettymys (disappointment):

pettymys = disappointment
pettymyksestä = from (a/the) disappointment

The verb oppia (to learn) normally takes the elative for the source of learning:

opin tästä virheestä = I learned from this mistake
opin paljon kurssilta = I learned a lot from the course

So:

mistä pettymyksestä opin eniten = “from which disappointment I learned the most”

The -sta / -stä ending expresses “from” here.

What case is päiväkirjaan, and why do we use that form with kirjoittaa?

Päiväkirjaan is the illative case of päiväkirja (diary):

päiväkirja = diary
päiväkirjaan = into (the) diary

The verb kirjoittaa in the sense of “write into something” typically uses the illative:

kirjoitan vihkoon = I write into a notebook
kirjoitan tietokoneelle (slightly different structure) = I type on the computer
kirjoitan päiväkirjaan = I write in / into (a) diary

So päiväkirjaan literally means “into the diary”, which in English is usually just “in (my) diary”.

In English I’d say “in my diary”. Why doesn’t Finnish say päiväkirjaani here?

Finnish can mark possession with a possessive suffix:

päiväkirja = diary
päiväkirjani = my diary (basic form)
päiväkirjaani = into my diary (illative + possessive suffix)

So a very natural variant would be:

Illalla kirjoitan päiväkirjaani… = “In the evening I will write in my diary…”

The sentence Illalla kirjoitan päiväkirjaan… is also correct and means “I’ll write in a/the diary”. In real use, the context often makes it obvious it’s your own diary, so the suffix is optional, but including -ni (my) is a bit more explicit and typical if you want to stress that it’s your personal diary.

Why is there no minä (I) in kirjoitan or opin? Is it left out by mistake?

It’s not a mistake; Finnish usually drops subject pronouns when they are obvious from the verb ending.

kirjoitan = I write / I will write
opin = I learn / I learned

The -n ending marks the first person singular. Adding minä is possible but usually only needed for emphasis or contrast:

Minä kirjoitan päiväkirjaan (I, as opposed to someone else, will write in the diary.)

Since there is no contrast here, minä is naturally omitted.

What does eniten mean exactly, and how is it formed?

Eniten means “the most” (in terms of amount/degree).

It’s the superlative adverb related to paljon (a lot, much):

paljon = a lot
enemmän = more
eniten = most

So:

opin eniten = I learn the most / I learned the most

In the sentence, mistä pettymyksestä opin eniten tänään = “from which disappointment I learned the most today.”

Could the word order in mistä pettymyksestä opin eniten tänään be changed? For example: mistä pettymyksestä tänään opin eniten?

Yes, Finnish word order is relatively flexible, and both are grammatically correct:

mistä pettymyksestä opin eniten tänään
mistä pettymyksestä tänään opin eniten

The default, neutral order in your sentence places tänään at the end, which is very natural. Moving tänään earlier can slightly change the focus (for example, contrasting today with some other day), but the basic meaning stays the same.

What you cannot do is break up mistä and pettymyksestä in a way that disconnects them from each other, because together they form the phrase “from which disappointment”. Those two belong together.

Could you say pettymyksestä, josta opin eniten tänään instead of mistä pettymyksestä opin eniten tänään? What’s the difference?

Yes, you could say:

Illalla kirjoitan päiväkirjaan pettymyksestä, josta opin eniten tänään.

Here:

pettymyksestä = from the disappointment
josta = from which

This structure assumes a specific, already known disappointment, and josta explicitly refers back to pettymyksestä.

Your original version:

… päiväkirjaan, mistä pettymyksestä opin eniten tänään.

is a bit more like:

“about which disappointment I learned the most today”

Both forms are grammatically correct. The josta-version feels a bit more explicitly “relative clause attached to a noun”, while the mistä pettymyksestä version is more like a free relative clause (“about which disappointment…”). In everyday speech and writing, both patterns occur.