Kun pääsin hotelliin, huomasin, että olin jättänyt hammasharjan kotiin.

Kun introduces a time clause here: when.

So:

• Kun pääsin hotelliin = When I got to the hotel

In Finnish, kun can also sometimes mean as, once, or even because in some contexts, but in this sentence it is clearly temporal: it tells you when the main action happened.

Pääsin comes from päästä, which often means:

• to get to
• to reach
• to manage to get somewhere
• sometimes to be able to go / gain access

So pääsin hotelliin feels like I got to the hotel or I made it to the hotel.

By contrast:

• menin hotelliin = I went to the hotel

That is more neutral and just describes movement.
Pääsin often highlights the successful arrival, not just the act of going.

Because hotelliin is in the illative case, which often means into or to a place.

• hotelli = hotel
• hotelliin = into/to the hotel

With verbs of motion, Finnish often uses the illative:

• menen kouluun = I go to school
• tulen kotiin = I come home
• pääsin hotelliin = I got to the hotel

So the -in ending shows movement toward that place.

Finnish usually leaves out subject pronouns when the verb ending already shows the person clearly.

For example:

• pääsin = I got
• huomasin = I noticed
• olin jättänyt = I had left

Because these forms already show 1st person singular, minä is not necessary.

You can include minä for emphasis, contrast, or clarity, but in ordinary Finnish it is often omitted.

Että means that and introduces a content clause.

• huomasin, että... = I noticed that...

So the sentence structure is:

• Kun pääsin hotelliin = time clause
• huomasin = main verb
• että olin jättänyt hammasharjan kotiin = what I noticed

In English, that is often optional:

• I noticed I had left...
• I noticed that I had left...

In Finnish, että is very common in this kind of sentence.

Olin jättänyt is the pluperfect (also called past perfect).

It is formed with:

• olla in the imperfect: olin = I was
• past participle of the main verb: jättänyt = left

So:

• olin jättänyt = I had left

This tense is used because the leaving happened before the noticing:

1. I left my toothbrush at home.
2. Later, I got to the hotel.
3. Then I noticed it.

That earlier past action is exactly what the pluperfect is for.

It comes from the verb jättää = to leave.

The active past participle is formed from the verb stem, and here it becomes:

• jättää → jättänyt (masculine/feminine distinction does not exist in Finnish)

In the pluperfect, Finnish uses:

• olin jättänyt = I had left
• olit jättänyt = you had left
• oli jättänyt = he/she had left

So jättänyt is the participle part of the pluperfect construction.

Because hammasharjan is the object in the genitive/accusative-like form used for a total object in an affirmative sentence.

• hammasharja = toothbrush
• hammasharjan = the toothbrush / a toothbrush as a completed object

Here the action is seen as complete:

• I had left the toothbrush at home

So Finnish uses the total object form:

• olin jättänyt hammasharjan kotiin

Compare:

• Luin kirjan. = I read the whole book.
• Luin kirjaa. = I was reading a book / some of the book.

Here, leaving the toothbrush is a completed event, so hammasharjan is natural.

Yes, this is a very common learner question.

Normally:

• kotiin = home / to home
• kotona = at home

But with the verb jättää, Finnish often uses kotiin in the sense of leave behind at home.

So:

• jättää jotain kotiin = to leave something at home

This is idiomatic Finnish. It does not mean you actively moved the toothbrush into the home in this sentence. It means it ended up being left behind there.

So although kotona means at home, after jättää the usual expression is:

• Unohdin kirjan kotiin. = I forgot the book at home.
• Jätin lompakon kotiin. = I left my wallet at home.

Yes. Finnish uses compound words very often.

• hammas = tooth
• harja = brush
• hammasharja = toothbrush

Notice that hammas changes slightly inside the compound:

• hammas → hammas- / hampaa- / hammash- depending on the word
• here it becomes hammas + harja → hammasharja

You do not write it as two separate words.

Because Finnish uses commas to separate subordinate clauses quite regularly.

Here the sentence has two subordinate clauses:

1. Kun pääsin hotelliin, ...
2. ..., että olin jättänyt hammasharjan kotiin.

So the commas help mark the clause boundaries.

Finnish comma usage is often more systematic than English in these clause structures, so learners should pay attention to them.

The structure is:

1. Kun pääsin hotelliin = subordinate time clause
2. huomasin = main clause verb
3. että olin jättänyt hammasharjan kotiin = subordinate content clause

So the full pattern is:

• When X happened, I noticed that Y had happened earlier.

Finnish word order is fairly flexible, but this version is very natural and neutral.

Yes. A few possible alternatives are:

• Kun saavuin hotelliin, huomasin, että olin jättänyt hammasharjan kotiin.
  = When I arrived at the hotel...
  Slightly more formal/literary than pääsin.

• Hotelliin päästyäni huomasin, että olin jättänyt hammasharjan kotiin.
  = After getting to the hotel, I noticed...
  More advanced structure.

• Kun tulin hotelliin, huomasin, että olin jättänyt hammasharjan kotiin.
  = When I came to the hotel...
  Possible, though pääsin hotelliin emphasizes successful arrival a bit better.

The original sentence is very natural everyday Finnish.

Here are the key dictionary forms:

• kun = when
• pääsin → päästä
• hotelliin → hotelli
• huomasin → huomata
• että = that
• olin → olla
• jättänyt → jättää
• hammasharjan → hammasharja
• kotiin → koti

This is useful because Finnish words often appear in inflected forms that look quite different from the dictionary form.