Learn Hardcore Finnish: Sunnuntaina kirjoitan päiväkirjaan, millainen työpäivä oli ja missä kohdassa selässä tuntui kipua. - On Sunday I write in the diary what the workday was like and where in the back I felt pain.

So Sunnuntaina kirjoitan… literally means “On Sunday I write…” (i.e. “I will write on Sunday”). Finnish doesn’t need a separate preposition like on; the case ending does the job.

Why is it kirjoitan päiväkirjaan and not kirjoitan päiväkirjassa or just kirjoitan päiväkirja?

Päiväkirjaan is illative case (movement “into” something).

päiväkirja = diary (nominative)
päiväkirjaan = into the diary

When you write into a diary, letter, notebook, etc., Finnish uses the illative:

kirjoitan kirjeen vihkoon = I write the letter into the notebook
kirjoitan ajatukseni päiväkirjaan = I write my thoughts in/into my diary

By contrast:

päiväkirjassa (inessive, “in the diary”) would describe something located inside the diary, not the act of writing into it.
You cannot say kirjoitan päiväkirja without a case ending; that would be ungrammatical. Objects and complements need a case.

So kirjoitan päiväkirjaan = “I write in/into (the) diary.”

Shouldn’t it be päiväkirjaani (“into my diary”) instead of päiväkirjaan?

Both are possible, but they differ slightly in emphasis.

päiväkirjaan = into (the) diary (context usually makes it clear it’s your own diary)
päiväkirjaani = into my diary (illative + possessive suffix -ni “my”)

If you especially want to highlight that it is your diary, päiväkirjaani is more explicit:

Sunnuntaina kirjoitan päiväkirjaani… = On Sunday I’ll write in my diary…

In many real-life contexts, people leave out the possessive suffix when it’s obvious whose diary it is, so päiväkirjaan is quite natural.

What exactly is millainen, and how is it different from minkälainen or mikä?

Millainen is an interrogative adjective meaning “what kind of”, “what sort of”.

millainen työpäivä = what kind of workday

About the alternatives:

millainen and minkälainen both mean “what kind of” and are largely interchangeable.
- millainen työpäivä
- minkälainen työpäivä
  Both are acceptable; millainen is very common in modern usage.
mikä on its own means “what / which” (not “what kind of”):
- Mikä työpäivä se oli? = Which workday was it? / What (which) day at work was it?
  This is a different question from “what was the workday like?”.

In this sentence, millainen introduces an indirect question:
millainen työpäivä oli = (I write) what kind of workday it was.

What kind of clause is millainen työpäivä oli ja missä kohdassa selässä tuntui kipua?

This whole part is an indirect question clause (in Finnish grammar: epäsuora kysymyslause). It is the object/complement of kirjoitan:

(Minä) kirjoitan päiväkirjaan, millainen työpäivä oli ja missä kohdassa selässä tuntui kipua.
= I write in my diary what kind of workday it was and where in my back I felt pain.

Inside that subordinate clause, there are two coordinated indirect questions joined by ja (“and”):

millainen työpäivä oli = what kind of workday it was
missä kohdassa selässä tuntui kipua = where in (my) back I felt pain

In Finnish, indirect questions start with question words like millainen, missä, mitä, miksi, etc., but do not take että (“that”). You cannot say että millainen työpäivä oli here.

Why is the word order millainen työpäivä oli and not oli millainen työpäivä?

Both orders are grammatically possible, but millainen työpäivä oli is the natural pattern that mirrors a direct question:

Direct question: Millainen työpäivä oli? = What was the workday like?
Indirect: Hän kertoi, millainen työpäivä oli.

In Finnish subordinate clauses, the verb does not have to come early as in English. It is common, especially in indirect questions, for the question word + subject + verb order to appear:

En muista, mitä hän sanoi. = I don’t remember what he said.
Kerro, missä hän asuu. = Tell me where he lives.

So millainen työpäivä oli is completely normal and idiomatic.

Why is kirjoitan in the present tense but oli and tuntui are in the past?

Finnish does not have a separate future tense, so present tense often covers future meaning:

Sunnuntaina kirjoitan… = On Sunday I will write…

The subordinate clause talks about events that are already in the past relative to that writing:

työpäivä oli = the workday was (already finished by Sunday)
tuntui kipua = (there) was pain / pain was felt (in the past)

So the timeline is:

Future relative to now: Sunnuntaina kirjoitan…
Past relative to that future Sunday: millainen työpäivä oli ja missä… tuntui kipua

This “mixed” tense pattern is normal and logical in Finnish.

In missä kohdassa selässä, why are there two location words — kohdassa and selässä — and why do they both have -ssa?

The phrase is built like this:

missä kohdassa = in which spot / at which point
- missä = in which (inessive of mikä)
- kohdassa = in the spot (inessive of kohta, “point, spot, place”)
selässä = in the back (inessive of selkä, “back”)

Together: missä kohdassa selässä = “in which spot in the back”.

All three use the inessive case (-ssa / -ssä) to indicate a static location:

missä? = in where?
kohdassa = in the spot / at the point
selässä = in the back

So you can think of it as: in which spot (located) in the back.

Why is it selässä and not selkään or something like selässäni?

Case:

selässä is the inessive case (inside / in something), answering missä? = where?
selkään would be illative (into something), answering mihin? = into where?

We want a static location of the pain (“where in my back the pain was”), so selässä is correct.

Possession:

selässä = in (the) back
selässäni = in my back (inessive + possessive suffix -ni)

Often, in contexts where we’re clearly talking about the speaker’s own body, Finnish may omit the possessive suffix, so selässä can still be understood as “in my back”. Adding -ni would simply make it more explicit:

missä kohdassa selässäni tuntui kipua = in which spot in my back I felt pain

Why is kipua in the partitive case instead of kipu?

Kipua is the partitive singular of kipu (“pain”).

With verbs describing sensations, experiences, and ongoing states, Finnish often uses the partitive object to express something indefinite, partial, or not fully bounded:

Tunsin kipua selässäni. = I felt (some) pain in my back.
Hänellä on päänsärkyä. = She has a headache / some head pain.

Compare:

Tunsin kivun selässäni. = I felt the pain in my back. (more specific, a defined instance)

In selässä tuntui kipua, the idea is “there was (some) pain felt in the back”, not a single, clearly delimited “pain event”. That’s why kipua (partitive) is natural.

What does tuntui kipua literally mean, and who is the “subject” here?

Literally, tuntui kipua is something like:

“pain was felt” / “there was a feeling of pain”.

Structure:

tuntua = to feel, to be felt, to seem
kipua (partitive) = (some) pain

The sentence selässä tuntui kipua can be loosely rendered as:

“(In the back) there was pain (felt).”

No explicit I or he/she is stated; the experiencer is understood from context (or could be added with another structure, e.g. Minulla selässä tuntui kipua). This impersonal style is common in Finnish for sensations and physical states.

Why is there a comma before millainen?

In Finnish, a comma is normally used between a main clause and a subordinate clause, including indirect questions:

Main clause: Sunnuntaina kirjoitan päiväkirjaan
Subordinate (indirect question): millainen työpäivä oli ja missä kohdassa selässä tuntui kipua

Therefore, a comma is placed before millainen:

Sunnuntaina kirjoitan päiväkirjaan, millainen työpäivä oli…

The whole part starting with millainen (and including ja missä…) functions as one subordinate clause. You do not add että before millainen; the question word itself (millainen/missä) is enough to mark it as an indirect question.