Learn Hardcore Danish: Lyset i gangen er klart om aftenen. - The light in the hallway is bright in the evening.

Common gender (en-word), singular: klar
- Lampen er klar. – The lamp is bright/ready.
Neuter gender (et-word), singular: klart
- Lyset er klart. – The light is bright.
Plural and definite: klare
- Lysene er klare. – The lights are bright.

Because lys is neuter (et lys), we need the neuter form klart:
Lyset … er klart …

Does the adjective always agree with the noun like this in Danish?

For adjectives that describe a subject with er (or another form of “to be”), yes, they normally agree in gender and number with that subject:

Lampen er klar. (common gender, singular)
Lyset er klart. (neuter, singular)
Lamperne er klare. (plural, definite)
Lyset i gangen er klart. (subject is still lyset, so klart)

So this agreement is something you must pay attention to in Danish, unlike in English.

Why is it i gangen and not something like på gangen?

Both i and på can translate to in / on, but they are not interchangeable.

i gangen literally: in the hallway / corridor (inside that space)
på gangen is possible in some contexts, but sounds more like “out in the hall” as an area outside rooms, or can have a slightly different nuance. In many everyday cases, i gangen is the default for location.

Also, note that i gang (without -en) is an idiom meaning “in progress / underway”, which is something completely different:

Arbejdet er i gang. – The work is underway.

Here we are clearly talking about a physical hallway, so i gangen is the natural choice.

Why is it gangen and not just gang?

Same reason as with lyset: gangen is the hallway, not just hallway.

en gang can mean a hallway, but it also very often means one time / once, which would be confusing here.
gangen = the hallway / the corridor (definite singular)

So:

i gangen = in the hallway (a specific hallway people know from context)

What does om aftenen mean literally, and why use om here?

Literally:

om = about / around / during (here: in / during)
aftenen = the evening (definite singular of aften)

In time expressions, om is commonly used for “in the [part of the day]” when speaking about something that happens habitually or generally:

om morgenen – in the morning(s)
om eftermiddagen – in the afternoon(s)
om aftenen – in the evening(s)
om natten – at night

So om aftenen here is “in the evening” in a general, repeated sense (not just this evening).

Could I say om aftenerne instead of om aftenen? What would be the difference?

Both are grammatically possible, but the nuance shifts slightly:

om aftenen – literally “in the evening” (as a part of the day in general)
- Typical way to talk about a usual routine:
  - Lyset i gangen er klart om aftenen. – The light is bright in the evening (generally).
om aftenerne – literally “in the evenings” (plural, the evenings)
- Emphasizes the individual evenings as repeated separate times:
  - Lyset i gangen er klart om aftenerne. – The light is bright in the evenings (as a recurring pattern).

In everyday speech, om aftenen is more common for a general statement like in this sentence.

Is the word order fixed, or can I move om aftenen to the front?

You can move om aftenen to the front, but then you must still keep the verb in the second position (the V2 rule):

Original: Lyset i gangen er klart om aftenen.
- Subject (Lyset i gangen) comes first, verb (er) second.

If you front the time expression:

Om aftenen er lyset i gangen klart.

Here:

Om aftenen (time expression) is in first position.
The verb er must still be second.
The subject lyset i gangen comes after the verb.

Both sentences are correct Danish; the second just emphasizes “In the evening” more.

Is there any special reason why i gangen comes before er and klart?

In the original sentence:

Lyset i gangen er klart om aftenen.

i gangen is part of the subject phrase:

Lyset i gangen = “the light in the hallway”

So the structure is:

Subject: Lyset i gangen
Verb: er
Predicative adjective: klart
Time adverbial: om aftenen

We are not just saying “the light” and then adding “in the hallway” later; we define which light we are talking about right in the subject itself: the light in the hallway.

How is the whole sentence pronounced in Danish?

Approximate standard Danish pronunciation (very roughly, in IPA-like transcription):

Lyset – [ˈlyːsəð] (the final t is silent; the d can be very soft or almost gone)
i – [i]
gangen – [ˈgɑŋən] (the g is hard; -en is a weak syllable)
er – [æɐ̯] (often almost like a short “air”)
klart – [klɑːɐ̯d̥] (final t often very weak or not clearly released)
om – [ʌm]
aftenen – [ˈaftənən] or [ˈɑfdənən] (the t can assimilate, becoming more like d in fast speech)

Spoken fairly naturally, the sentence can sound something like:

[ˈlyːsəð i ˈgɑŋən æɐ̯ klɑːɐ̯d̥ ʌm ˈaftənən]

Don’t worry about copying this perfectly; focusing on vowel length and stress will already help a lot.

Does klart here mean “clear” or “bright”? Could I also use another word?

Klart can mean both clear and bright, depending on context.

For light, klart lys or lyset er klart usually suggests that the light is strong/bright, not dim or weak.

You could also say, for example:

Lyset i gangen er stærkt om aftenen. – The light in the hallway is strong in the evening.
Lyset i gangen er meget lyst om aftenen. – The light in the hallway is very bright in the evening.

But klart is natural and idiomatic in this sentence.

Could Danish use something like “is being bright” here (a progressive form), or is er always just “is”?

Danish does not have a special present continuous form like English “is being”.
The simple present er covers both “is” and contexts where English might use “is being”.

So:

Lyset i gangen er klart om aftenen.
can only be expressed this way; you do not say “er ved at være klart” or similar for a normal, general statement like this.

For a general fact or habitual situation, simple er is always the right choice.