Learn Hardcore Norwegian: I mørketiden henger det små reflekser på ryggsekkene deres. - In the dark season there are small reflectors hanging on their backpacks.

You can say i den mørke tiden, but that sounds more descriptive and general, like “in the dark time” (could be any dark time, even metaphorical).

i mørketid without the definite ending is not idiomatic in this meaning; it sounds wrong to native speakers in this context.

What exactly does mørketiden refer to?

Literally it is “the dark time/season”, but culturally it usually refers to the part of winter in the far north of Norway when:

the sun never rises above the horizon (polar night), or
at least it is very dark for most of the day

So i mørketiden evokes that specific, well-known dark period of the year, not just “when it’s dark” in general.

Why is the word order henger det små reflekser and not det henger små reflekser?

In fact, both word orders are possible:

I mørketiden henger det små reflekser på ryggsekkene deres.
I mørketiden det henger små reflekser på ryggsekkene deres. ❌ (this one is not allowed)
Det henger små reflekser på ryggsekkene deres i mørketiden. ✅

What’s going on is the V2 rule in Norwegian: in a main clause, the finite verb must be in second position.

In your sentence:

I mørketiden = first element (adverbial / time phrase)
Therefore the verb henger must come next (second)
Then we get det små reflekser på ryggsekkene deres

So the order is:
[I mørketiden] – [henger] – [det] – [små reflekser] – [på ryggsekkene deres]

What is the function of det in henger det små reflekser? Why not de since reflekser is plural?

Here det is a dummy/expletive subject, like “there” in English:

Det henger små reflekser på ryggsekkene deres.
≈ There are small reflectors hanging on their backpacks.

It does not refer to anything, so it does not need to agree in number with reflekser.

If you wanted a real pronoun referring to the plural reflekser, then you would use de, but then the structure would be different, for example:

De henger på ryggsekkene deres. – They are hanging on their backpacks. (here de clearly refers to reflekser)

In the original sentence, det is just a grammar placeholder to say “there are …”.

Could we say Små reflekser henger på ryggsekkene deres instead? Is that different?

Yes, that is perfectly correct, but it has a slightly different feel.

Det henger små reflekser på ryggsekkene deres.
– More existential: you are introducing the fact that such things exist there (similar to “There are small reflectors hanging on their backpacks.”).
Små reflekser henger på ryggsekkene deres.
– Sounds a bit more descriptive/listing: you are directly stating what hangs there (like “Small reflectors hang on their backpacks.”).

Both are grammatical. The original with det is very natural when you want to introduce something new to the listener.

Why is henger used here instead of er? Could we say I mørketiden er det små reflekser på ryggsekkene deres?

henger is the normal verb because the reflectors are physically hanging from the backpacks. It describes how they are attached.

You could say:

I mørketiden er det små reflekser på ryggsekkene deres.

This is also grammatical, but now the focus is just “there are small reflectors on their backpacks”, without specifying that they are hanging. It’s more about existence/location than the manner of attachment.

So:

henger – emphasizes the hanging position
er – just states that they are present/there

What are reflekser in Norwegian? Does it just mean “reflections”?

In ordinary everyday use, en refleks / reflekser usually means:

small reflective items you attach to clothes, bags, etc. so drivers can see you in the dark

For example:

å bruke refleks – to wear a reflector (for visibility)
refleksbrikke / refleksvest – reflector tag / reflective vest

The more general meaning “reflection” (as in physics or philosophy) is less common in daily speech; in the context of backpacks and the dark season, it clearly means safety reflectors.

Why is it små reflekser and not something with liten like liten refleks?

The base adjective is liten (small), but it changes form:

en liten refleks – a small reflector (masc./fem. singular, indefinite)
et lite hus – a small house (neuter singular, indefinite)
små reflekser – small reflectors (plural)
små hus – small houses (plural)

So:

liten / lite = singular
små = plural

Because reflekser is plural, you must use små: små reflekser.

Why is it ryggsekkene (definite plural) when we already have deres? Isn’t that “double” marking?

Yes, this is the normal “double definiteness” pattern in Norwegian when the possessive comes after the noun.

The pattern is:

ryggsekker – backpacks (indefinite plural)
ryggsekkene – the backpacks (definite plural)
ryggsekkene deres – their backpacks

So in this structure (noun + definite ending + possessive after the noun), Norwegian requires the definite suffix -ene:

på ryggsekkene deres – on their backpacks

Without -ene, på ryggsekker deres sounds ungrammatical in standard Norwegian.

Could we say på deres ryggsekker instead of på ryggsekkene deres?

You can say på deres ryggsekker, but it has a different structure and feel:

på ryggsekkene deres
- noun is definite plural (ryggsekkene)
- possessive comes after the noun
- very common, neutral: simply “on their backpacks”
på deres ryggsekker
- noun is indefinite plural (ryggsekker)
- possessive comes before the noun
- often has a slight emphasis on deres = “on their backpacks” (as opposed to someone else’s)

So the original på ryggsekkene deres is the most natural neutral wording here.

Why is it deres and not sine in på ryggsekkene deres?

Norwegian has two kinds of possessives:

deres – non-reflexive (their / your-plural-formal)
sin / si / sitt / sine – reflexive possessive (his/her/their own)

Reflexive possessives (sin / si / sitt / sine) refer back to the subject of the clause. But in this sentence, the grammatical subject is the dummy det:

I mørketiden henger det små reflekser på ryggsekkene ...

Since det is just a dummy subject and does not refer to a group of people, you cannot use sine here. You must use deres:

på ryggsekkene deres – on their backpacks

If you changed the sentence so that they were the subject, then you could use sine:

I mørketiden har de små reflekser på ryggsekkene sine.
– de (they) = subject
– sine clearly refers back to de

Can deres mean both “their” and “your (plural/formal)” in Norwegian?

Yes. Deres can mean:

their – third person plural possessive
- barna deres – their children
your (plural, polite/formal) – addressing several people formally
- Deres bil står i veien. – Your car is in the way. (formal, plural or polite)

In most contexts, the meaning is clear from the situation and the rest of the sentence.
In på ryggsekkene deres, with no direct address form, it will normally be understood as “their backpacks”, not “your backpacks”.