Glas og metal skal sorteres for sig.

Here skal expresses obligation or requirement.

So in this kind of sentence, skal usually means something like:

• must
• has to
• sometimes should, depending on how strict the context is

On a recycling sign or instruction, skal is normally stronger than a friendly suggestion. It means this is the rule.

Because after a modal verb like skal, Danish uses the infinitive of the main verb.

• active infinitive: sortere = to sort
• passive infinitive: sorteres = to be sorted

So:

• skal sortere = must sort
• skal sorteres = must be sorted

That means skal sorteres is a very normal way to say must be sorted.

The -s marks the passive voice.

So:

• sortere = sort
• sorteres = be sorted / is sorted / are sorted, depending on context

In this sentence, the passive is used because the focus is on what must happen to glass and metal, not on who does it.

This is very common in Danish instructions, rules, and signs.

Danish actually has two common ways to make the passive:

• s-passive: sorteres
• blive-passive: bliver sorteret

So Danish could say:

• Glas og metal skal sorteres for sig
• Glas og metal skal blive sorteret for sig

But the first version with -s is usually shorter, more neutral, and more natural on signs and official instructions.

English speakers often expect a separate auxiliary verb, but Danish often does not need one.

For sig is an idiomatic expression meaning:

• separately
• on its own
• apart

In waste-sorting language, it means something should be kept in a separate category or not mixed in with other things.

So sorteres for sig means sorted separately.

Sig is the Danish reflexive pronoun used in expressions like for sig.

In this sentence, for sig is best learned as a set phrase meaning separately.

A native English speaker might try to analyze sig word-for-word, but in practice it is more useful to remember:

• for sig = separately
• hver for sig = each separately

So yes, sig is a reflexive pronoun, but the whole phrase is what really matters here.

Because glas and metal are being used as general material/category words, not as specific individual objects.

This is very common in Danish, especially on signs and labels.

So here:

• glas = glass as a material/category
• metal = metal as a material/category

If you added articles, the meaning would become more specific:

• et glas = a glass (for drinking)
• glasset = the glass
• metallet = the metal / a specific metal item

But in recycling instructions, Danish usually uses the bare category word.

Here it is not really functioning as an ordinary countable singular or plural noun. It is being used more like a material noun or category label.

That is similar to English:

• Glass and metal must be sorted separately

In that English sentence, glass and metal are also category words, not ordinary counted items.

So in this Danish sentence, it is best to think of glas and metal as waste categories, not as singular objects.

Not necessarily.

This sentence can mean that glass and metal belong in a separate sorting group, as opposed to being mixed with general waste.

Whether they must be separated from each other depends on the real-world context, such as the local recycling system.

If Danish wants to make it clearer that each one must be separate, it often uses:

• hver for sig = each separately

For example:

• Glas og metal skal sorteres hver for sig.

That more clearly suggests glass separately, metal separately.

So for sig can sometimes be a little broader than English learners expect.

Yes, you could say:

• Glas og metal skal sorteres separat.

That is understandable and grammatical.

But there is a small difference in feel:

• for sig is very common and idiomatic in practical instructions
• separat is also correct, but can sound a little more formal or less idiomatic in some contexts
• hver for sig is the best choice if you want to stress individually / one by one / not together

So all three can work, but they are not always exactly the same.

Because Danish signs and rules often use a neutral, impersonal style.

So instead of saying:

• Du skal sortere glas og metal for sig.
  (You must sort glass and metal separately.)

Danish often prefers:

• Glas og metal skal sorteres for sig.

This sounds more like a general rule or instruction, and less like someone speaking directly to one person.

That style is very common in:

• notices
• public signs
• official instructions
• labels
• recycling rules