Learn Hardcore Swedish: På hösten känns de mörka kvällarna längre än på sommaren. - In the autumn the dark evenings feel longer than in the summer.

Here we’re talking about how evenings are perceived, not about their actual clock length. So Swedish uses känns to signal this subjective feeling, just as English uses feel:

Det känns kallt. – It feels cold. (Maybe it’s not objectively that cold.)
Kvällarna känns längre. – The evenings feel longer. (They seem to drag on.)

Saying Kvällarna är längre would mean the evenings really are longer in terms of time, which is usually not what is meant here.

Why do we say "på hösten" and "på sommaren" instead of "i hösten" or just "hösten"?

With seasons, Swedish normally uses på when speaking about something that generally happens during that season:

på hösten – in (the) autumn / in fall
på vintern – in (the) winter
på sommaren – in (the) summer
på våren – in (the) spring

So:

På hösten känns de mörka kvällarna längre.
= In autumn, the dark evenings feel longer.

"i hösten" is not idiomatic in this general sense.

Compare:

i höst = this coming autumn (specific time period)
- Vi ses i höst. – We’ll see each other this autumn.

So:

på + definite season = in general, every year
i + season in bare form (i höst, i sommar) = this coming season

Why are hösten, sommaren, and kvällarna in the definite form?

Swedish uses the definite form more often than English, especially for:

Seasons in a general sense
Saying på hösten, på sommaren with definite forms is the normal way to talk about autumn/summer in general, not just one particular autumn or summer.
Specific sets of things that speaker and listener can identify
de mörka kvällarna = the dark evenings – specifically those in autumn that we’re talking about.

Forms here:

hösten = the autumn/fall (definite singular)
sommaren = the summer (definite singular)
kvällarna = the evenings (definite plural)

English often uses no article in similar general statements (“In summer…”, “In winter…”), but Swedish typically uses the definite here.

What is the structure of "de mörka kvällarna"? Why de, mörka, and -arna?

de mörka kvällarna is a definite plural noun phrase:

de – definite article, plural (for common-gender nouns)
mörka – adjective mörk (dark) in definite plural form (-a ending)
kvällarna – noun kväll (evening) in definite plural (-arna ending)

Indefinite vs definite:

en kväll – an evening
flera kvällar – several evenings (indefinite plural)
de mörka kvällarna – the dark evenings (definite plural)

Adjective agreement:

Indefinite singular common gender: en mörk kväll – a dark evening
Indefinite plural: mörka kvällar – dark evenings
Definite plural: de mörka kvällarna – the dark evenings

So the pattern is:

de + mörk-a + kväll-ar-na
article + adjective with -a + plural noun with definite -na/-arna

How is "de" pronounced here, and what about "dem" and "dom"?

In modern standard speech:

de (subject form) is almost always pronounced [dom]
dem (object form) is also pronounced [dom]

So de mörka kvällarna is normally said:

dom mörka kvällarna

In writing, however, you should keep the distinction:

de – subject:
- De mörka kvällarna känns längre. – They feel longer.
dem – object:
- Jag gillar dem. – I like them.

dom is used informally in text messages, social media, etc., but in standard written Swedish you write de/dem, even though both are pronounced dom.

Is "känns" a passive form? How is it related to "känna"?

The -s ending can mark passive voice in Swedish, but some verbs with -s are “s‑verbs” with their own meaning, not just passives.

känna = to feel (actively), to know (a person, a place)
- Jag känner honom. – I know him.
kännas = to feel, to be experienced as (no object; more like “to seem/feel”)
- Det känns bra. – It feels good.
- Kvällarna känns längre. – The evenings feel longer.

So känns here is:

3rd person singular present of kännas
Not a passive of känna in this sentence
Used like English “feel / seem” about how something is perceived

Why do we repeat "på" in "än på sommaren"? Could we say "än sommaren" instead?

In comparisons, Swedish uses än = than.

Here we’re comparing how evenings feel in two different seasons, so each time expression keeps its own preposition:

på hösten … än på sommaren
= in autumn … than in summer

If you said:

… längre än sommaren

it would literally mean something like “longer than the summer (itself)”, which sounds odd and changes the meaning.

So:

längre på hösten än på sommaren – natural
längre på hösten än sommaren – not idiomatic for this meaning

Why is the present tense (känns) used here?

The present tense in Swedish is often used for:

General truths, habits, or repeated situations

Here, känns describes something that typically happens every autumn, not just once:

På hösten känns de mörka kvällarna längre…
= (In general) In autumn, dark evenings feel longer…

This is similar to English:

“In winter the days are shorter.”
“In summer it gets hot.”

So the present tense expresses a general, timeless observation, not “right now at this exact moment.”

Do seasons take capital letters in Swedish, like in English?

No. In Swedish, seasons are not capitalized (unless they begin a sentence):

hösten – autumn / fall
sommaren – summer
vintern – winter
våren – spring

So:

På hösten känns de mörka kvällarna längre än på sommaren.

is correctly written with lowercase for both hösten and sommaren.

What is the gender and plural of kväll, and why specifically "kvällarna"?

Kväll is a common-gender noun (an en-word):

Singular:
- en kväll – an evening
- kvällen – the evening
Plural:
- kvällar – evenings
- kvällarna – the evenings

In the sentence, we’re talking about evenings in general during autumn, as a recognizable group:

de mörka kvällarna = the dark evenings (those autumn evenings)

So we need definite plural, which is kvällarna.