Learn Hardcore Finnish: Takki on eteisessä. - The coat is in the hallway.

Takki on eteisessä = The coat is in the hallway.
You’d see other forms in other roles:
takin (genitive) often marks possession or certain objects: Takin väri on musta = The coat’s color is black.
takkia (partitive) appears with incomplete/ongoing actions or some verbs: Etsin takkia = I’m looking for a coat / looking for the coat.

What does on mean here, and why is it always the same form?

on is the 3rd person singular form of the verb olla = to be.
Finnish conjugates olla by person:

minä olen = I am
sinä olet = you are
hän on = he/she is
me olemme = we are
te olette = you (pl) are
he ovat = they are

Here the subject is takki (3rd person singular), so you use on.

Why is eteisessä in that form? What case is it?

eteisessä is in the inessive case, which roughly means in or inside something.

Base word: eteinen = hallway/entryway
Inessive singular: eteisessä = in the hallway

Finnish often expresses location with cases instead of separate words like in/at/on.

How do I know when to use -ssa/-ssä (inessive) instead of something like -lla/-llä?

A useful beginner rule:

-ssa/-ssä (inessive) = in/inside (an enclosed space)
-lla/-llä (adessive) = on/at (a surface or a general “at someone’s place” idea)

Examples:

Takki on eteisessä. = in the hallway (a space)
Takki on tuolilla. = on the chair (surface)
Takki on Matilla. = Matt has the coat / the coat is with Matt

Some places take adessive even if they feel like “places” (e.g., asemalla “at the station”), so there’s also idiomatic usage to learn.

Why is it eteisessä (with -ssä) and not eteisessa (with -ssa)?

Finnish has vowel harmony. Words with front vowels ä, ö, y usually take front-vowel endings like -ssä, -llä, -stä.
Since eteinen contains e, i (neutral vowels) and historically patterns with front harmony in this form, you get:

eteisessä (not eteisessa)

As a contrast:

talo → talossa (back vowel a, o, u → -ssa)
kylä → kylässä (front vowel ä → -ssä)

Why does the stem change from eteinen to eteis- in eteisessä?

That’s a common Finnish stem alternation type. Many nouns ending in -nen change to -s- in most inflected forms:

eteinen → eteis-
- -ssä → eteisessä
  Other examples:
nainen → naisessa (in the woman / in a woman—as a form example)
suomalainen → suomalaisessa (in a Finn / in the Finnish person, form example)

So: -nen → -s- is a pattern worth memorizing.

Is this sentence definite or indefinite? Does it mean a coat or the coat?

Finnish has no articles like a/the, so Takki on eteisessä can mean either a coat or the coat, depending on context.
Context clues decide:

If you’re talking about a specific coat you both know: “The coat is in the hallway.”
If you’re describing what’s in the hallway in general: “There is a coat in the hallway.”

What is the difference between Takki on eteisessä and a “there is” sentence?

Takki on eteisessä is a normal copular sentence: The coat is in the hallway.
Finnish also has an existential pattern that often corresponds to “there is/are”:

Eteisessä on takki. = There is a coat in the hallway.

Word order helps signal information structure:

Takki first = the coat is the topic (known/given)
Eteisessä first = the hallway is the setting, and you’re introducing what’s there

Both are grammatical; they just emphasize different things.

Is the word order flexible? Could I say On takki eteisessä?

Finnish word order is fairly flexible, but not all orders are equally natural. Common options:

Takki on eteisessä. (neutral “the coat is in the hallway”)
Eteisessä on takki. (existential “there is a coat in the hallway”)

On takki eteisessä is possible but usually sounds marked—like you’re answering a question or correcting someone (e.g., “It is a coat in the hallway,” contrastive focus). In normal neutral statements, you’d typically prefer one of the first two.

How would I make it negative?

Use the negative verb ei + the connegative form of the main verb:

Takki ei ole eteisessä. = The coat is not in the hallway.

Here ole is the connegative form of olla used after ei.

How do I ask a yes/no question with this sentence?

Add -ko/-kö to the verb (or the focused word). Most neutral:

Onko takki eteisessä? = Is the coat in the hallway?

You can also focus other parts:

Takki onko eteisessä? (less neutral; usually not preferred)
Eteisessäkö takki on? = Is it in the hallway that the coat is? (emphasis/contrast)

How would I ask Where is the coat?

Use missä = where (location, “in/at”):

Missä takki on? = Where is the coat?

And the answer matches the inessive location:

Takki on eteisessä.

What’s the difference between eteinen and eteinen/eteisessä in English—hallway, entryway, vestibule?

Eteinen usually refers to the entry area of a home/apartment: the space just inside the front door where you might leave shoes and coats. Depending on the building, English translations vary:

entryway, hall, hallway, foyer
So eteisessä is “in the entryway/hall.”

How do I say “into the hallway” or “out of the hallway” with eteinen?

Finnish uses different local cases:

into = illative: eteiseen = into the hallway/entryway
- Laitan takin eteiseen. = I’ll put the coat into the hallway.
out of = elative: eteisestä = out of the hallway
- Otan takin eteisestä. = I’ll take the coat out of the hallway.

So:

eteisessä = in (inessive)
eteiseen = into (illative)
eteisestä = out of (elative)

If I want to specify “my coat,” what changes?

You typically use the genitive for “my” with a pronoun:

Minun takki on eteisessä. (understandable, but often corrected)
More natural is:
Minun takkini on eteisessä. = My coat is in the hallway.

Here takkini includes the possessive suffix -ni = my. In everyday speech, Finns may also drop the suffix:

Mun takki on eteisessä. = My coat is in the hallway. (spoken, informal)

Does Finnish ever need an extra word like “in” here?

Not usually. Finnish normally encodes “in/at/on/from/to” with cases on the noun:

eteisessä already includes the meaning “in the hallway,” so adding a separate “in” word is unnecessary.
There are postpositions/prepositions in Finnish, but for basic locations, case endings are the main tool.