Learn Hardcore Finnish: Eteisessä on niin pimeää, että en löydä avainta. - It is so dark in the hallway that I can’t find the key.

Eteisessä is eteinen in the inessive case (ending -ssa/-ssä), meaning in the hallway/entryway.
So eteisessä = in the hall.

Why is the sentence using on without an explicit subject (no “it”)?

Finnish often uses an existential/locational structure: [place] + on + [state/thing].
Here, Eteisessä on pimeää literally means In the hallway there is darkness / it’s dark in the hallway. Finnish doesn’t need a dummy subject like English it.

Why is pimeää in the partitive (-ä/-a) instead of nominative like pimeä?

In this construction, many weather/condition/state words appear in the partitive to express an ongoing, non-countable state:

on pimeää = it’s dark (darkness as an unbounded condition)
Compare:
on pimeää (general darkness)
on pimeä can occur in more specific contexts, but on pimeää is the normal choice for “it’s dark”.

How does niin … että work?

niin … että is a common Finnish pattern meaning so … that.

niin pimeää, että … = so dark that …
The first part sets the degree (niin pimeää), and the second part gives the result (että en löydä avainta).

Why is there a comma before että?

Because että introduces a subordinate clause (a dependent clause). In Finnish, it’s standard to put a comma before most subordinate clauses:
… niin pimeää, että en löydä …

Why does the second clause start with en?

Finnish forms negation with a negative auxiliary verb that conjugates for person/number:

en = I do not
et = you do not
ei = he/she/it does not
Then the main verb appears in a special form (often called the connegative):
en löydä = I don’t find

What form is löydä in en löydä?

It’s the present tense connegative form used after the negative verb.
Positive: minä löydän = I find
Negative: minä en löydä = I don’t find

Why is avain in the form avainta (partitive) instead of avaimen or avaimen-like “the key”?

In Finnish, the object case changes with negation: a direct object in a negative clause is typically partitive.
So:

Positive: Löydän avaimen. = I find the key. (often genitive/accusative-type object)
Negative: En löydä avainta. = I don’t find the key. (partitive)

Could you also say en löydä avaimeni (my key) or avaimia (keys)?

Yes, and the object marking follows the same logic:

En löydä avaintani. = I can’t find my key. (partitive singular)
En löydä avaimia. = I can’t find (any) keys. (partitive plural)

Does niin pimeää describe the hallway or the general situation?

It describes the conditions in the hallway. Finnish makes this clear by putting the location first: Eteisessä on …. The darkness is presented as something that exists/prevails in that place.

Is eteinen always “hallway”?

Eteinen usually means an entryway/foyer—the space just inside the front door where you might hang coats and leave shoes. In many homes it functions like a hallway, but the nuance is the entrance area.

What should I pay attention to in pronunciation here?

Key points:

Eteisessä: stress on the first syllable: E-te-i-ses-sä (Finnish stress is almost always on the first syllable).
Long vowels matter: pimeää ends with a long ää sound (ää is longer than ä).
että has double t: hold the t slightly longer than a single t (a “long” consonant).