Learn Hardcore Finnish: Olen odotushuoneessa täyttämässä lomaketta, kun hoitaja kutsuu minut sisään. - I am in the waiting room filling in a form when the nurse calls me in.

Finnish uses case endings instead of separate prepositions like in, on, at.
So instead of saying “in the waiting room” with an extra word for in, Finnish just adds -ssa to the noun.

Why is it täyttämässä and not täytän in olen odotushuoneessa täyttämässä lomaketta?

Täyttämässä is the 3rd infinitive in the inessive (-massa/-mässä), used with olla and some movement verbs to mean “in the middle of doing something”.

Pattern:

olla + V-massa/V-mässä → “to be doing / to be in the process of doing”

Here:

täyttää = to fill in/out
täyttämä + ssä → täyttämässä = (being) in the act of filling in

So:

Olen täyttämässä lomaketta ≈ “I am in the middle of filling in a form.”

If you said:

Täytän lomaketta = I am filling in a form (simple present, can also describe ongoing action)
Olen täyttämässä lomaketta adds a nuance that your current activity or engagement at that place is filling in the form. It often answers the question “What are you (there) doing?”

What exactly does täyttämässä lomaketta express here: purpose, or ongoing action?

In this sentence it mainly expresses ongoing action / current activity.

Olen odotushuoneessa täyttämässä lomaketta =
I am in the waiting room, (there) in the middle of filling out a form.

This V-massa/V-mässä structure can express:

Current activity at a place
- Olen kirjastossa lukemassa. = I am (at) the library reading.
Purpose of going somewhere (with verbs like mennä, tulla, käydä)
- Menin kirjastoon lukemaan. = I went to the library to read.

Here, with olen, the primary idea is: “My current ongoing activity in the waiting room is filling out a form.”

Why is it lomaketta and not lomakkeen after täyttämässä?

Lomaketta is the partitive singular of lomake.

lomake (nom.) → lomaketta (partitive)
The partitive here marks an ongoing / not-yet-completed object.

With many verbs, especially when describing an action that is in progress, Finnish tends to use the partitive object:

Olen täyttämässä lomaketta.
= I’m (in the process of) filling in a form.
(The form-filling is not finished at that moment.)

Compare:

Täytän lomakkeen.
= I (will) fill in the form / I fill in the (whole) form.
(Focus on completing the entire form – lomakkeen is a total object.)

So täyttämässä lomaketta fits well with the idea of being in the middle of an ongoing action.

Why is there a comma before kun?

In Finnish, you normally put a comma between a main clause and a subordinate clause, even when the subordinate clause comes second.

Olen odotushuoneessa täyttämässä lomaketta, kun hoitaja kutsuu minut sisään.

Here:

Main clause: Olen odotushuoneessa täyttämässä lomaketta
Subordinate clause (introduced by kun): kun hoitaja kutsuu minut sisään

Rule of thumb:
If you can ask a question from the main clause and the subordinate clause answers when/why/how/although/etc., there should usually be a comma in Finnish.

What does kun do in kun hoitaja kutsuu minut sisään? Is it always just “when”?

In this sentence kun means “when” and introduces a time clause:

kun hoitaja kutsuu minut sisään = when the nurse calls me in

Kun can have a couple of main uses:

Temporal “when” (both single events and repeated situations)
- Kun tulen kotiin, syön. = When I come home, I eat.
Causal “since/because” in some contexts
- En voi tulla, kun olen kipeä. = I can’t come, since/because I’m ill.

Here, the meaning is clearly temporal: it tells at what moment something happens.

Why is it hoitaja without any article? How do I know if it is “a nurse” or “the nurse”?

Finnish does not have articles like a/an or the. The bare noun hoitaja can mean “a nurse” or “the nurse”, depending on context.

In this sentence:

kun hoitaja kutsuu minut sisään
will usually be understood as “when the nurse calls me in”, because in the typical context (a clinic or hospital) there is an obvious, specific nurse whose job it is to call patients in.

Definiteness/indefiniteness in Finnish is expressed by:

Context and shared knowledge
Sometimes word order
Occasionally adjectives like eräs (a certain) for explicitly indefinite, e.g. eräs hoitaja = a (certain) nurse.

So you have to infer a/the from the situation, not from a separate word.

Why is it minut and not minua after kutsuu?

Minut is the accusative form of minä, used for a total object.
Minua is the partitive form.

minä (I)
minut (me, as a whole object – accusative)
minua (me, partitive – partial/ongoing/etc.)

In kutsua joku sisään (“to call someone in”), the person is treated as a total object:

hoitaja kutsuu minut sisään = the nurse calls me (fully) in, once; the action achieves a clear result (I end up inside).

If you used minua, the meaning changes:

hän kutsuu minua
= he/she calls me (that name) / he/she is calling me (repeatedly, on the phone, etc.),
or with a complement:
He kutsuvat minua hulluksi. = They call me crazy.

So:

minut → I am the complete object of this one-off action of being called in.
minua → would imply a different construction/meaning.

Why does the sentence use present tense (olen, kutsuu) and not past tense (olin, kutsui) if I want to talk about a past event?

Finnish, like English, can use the present tense to narrate past events in a vivid way. This is similar to the “historical present” in English:

English: I’m sitting in the waiting room filling out a form, when the nurse calls me in.
Finnish: Olen odotushuoneessa täyttämässä lomaketta, kun hoitaja kutsuu minut sisään.

If you want to state it as a straightforward past event, you can absolutely use the past tense:

Olin odotushuoneessa täyttämässä lomaketta, kun hoitaja kutsui minut sisään.
= I was in the waiting room filling out a form when the nurse called me in.

So:

present + present → either actually happening now, or vivid storytelling of a past event
past + past → neutral narration of a past event

It’s best to keep the tenses consistent between the main clause and the kun-clause.

What is the difference between sisään, sisälle, and sisällä, and why is sisään used here?

These are related location/direction words built on sisä- (“inside”):

sisään: to the inside, in(wards) – direction in
sisälle: also “in(to)”, very similar to sisään, often slightly more concrete or colloquial
sisällä: inside – being in (no movement)

In this sentence:

kutsuu minut sisään = calls me in(wards), across the threshold and into the room.

You would typically say:

tulla sisään = to come in
mennä sisään = to go in
olla sisällä = to be inside

You could also hear:

kutsuu minut sisälle, which is not wrong; the difference between sisään and sisälle here is minimal. But the fixed phrase tulla/kutsua sisään is very common.

Can I change the word order, for example Kun hoitaja kutsuu minut sisään, olen odotushuoneessa täyttämässä lomaketta? Does it change the meaning?

Yes, that word order is perfectly correct:

Kun hoitaja kutsuu minut sisään, olen odotushuoneessa täyttämässä lomaketta.

Meaning-wise it’s the same. The difference is mostly information structure / emphasis:

Olen odotushuoneessa täyttämässä lomaketta, kun hoitaja kutsuu minut sisään.
→ Starts by setting the scene (where you are, what you’re doing), then tells what happens.
Kun hoitaja kutsuu minut sisään, olen odotushuoneessa täyttämässä lomaketta.
→ Starts with the time condition, then says what is true at that time.

Finnish word order is relatively flexible, especially with main + subordinate clauses. Just remember to keep the comma between them.