Vi skal folde papkassen sammen.

Skal is the present tense of skulle. In this sentence it expresses obligation or necessity, so it means something like have to, need to, or must.

So Vi skal folde papkassen sammen is most naturally understood as We have to fold the cardboard box up or We need to fold the cardboard box up.

Depending on context, skal can also suggest a planned future action, but obligation is the most useful reading here.

Because skal is a modal verb.

After Danish modal verbs such as skal, kan, vil, må, and bør, the next verb appears in the infinitive without at.

So:

• Vi skal folde ... = correct
• Vi skal at folde ... = incorrect

This is similar to English:

• We must fold ... not
• We must to fold ...

Here sammen does not mainly mean together in the social sense. With folde, it often works like a verb particle and gives the meaning fold up, fold together, or collapse.

So folde papkassen sammen means something like:

• fold the cardboard box up
• fold the cardboard box flat
• collapse the cardboard box

In other contexts, sammen really can mean together, so learners often notice this word and wonder about it. In this sentence, the most likely meaning is the result of the action: the box becomes folded up.

Because Danish, like English, often forms compound nouns by joining words together.

Here:

• pap = cardboard
• kasse = box

So:

• papkasse = cardboard box

Then the definite ending is added:

• papkassen = the cardboard box

This is very common in Danish. Compound nouns are usually written as a single word.

The ending -en marks the noun as definite.

Compare:

• en papkasse = a cardboard box
• papkassen = the cardboard box

This is different from English, where you normally put the in front. In Danish, the definite article is often attached to the end of the noun instead.

So:

• kasse = box
• kassen = the box

And in the compound:

• papkasse = cardboard box
• papkassen = the cardboard box

Because the normal definite form of a noun in Danish is usually the ending on the noun itself.

So the neutral way to say the cardboard box is:

• papkassen

Using den is usually for special situations, especially:

1. when there is an adjective
   
   • den store papkasse = the big cardboard box
2. when you are pointing something out with extra emphasis
   
   • den papkasse can mean something more like that cardboard box

So in this sentence, papkassen is the natural standard form.

Because Danish often places the object before the particle in this kind of verb phrase.

So:

• folde papkassen sammen is the normal order

This is similar to English:

• fold the box up

The particle sammen belongs with folde, but the noun object papkassen comes in between. That word order is very common and natural in Danish.

Grammatically, skal is in the present tense, but the meaning is not simply a present action like we are folding.

Instead, skal expresses something that is required, expected, or planned. So the sentence usually refers to an action that needs to happen now or soon.

A good way to think of it is:

• We have to fold the cardboard box up
• We need to fold the cardboard box up

So it is present in form, but it often points to a future or intended action.

Yes, it could, depending on context, but that is usually not the first interpretation in this sentence.

Without extra context, Vi skal folde papkassen sammen is most naturally understood as We need to fold the cardboard box up/collapse it.

If you wanted to make the idea of doing it jointly clearer, Danish would often use context, stress, or another expression such as sammen placed differently or i fællesskab.

So the word sammen can mean together, but here it most likely describes the folding result rather than the people doing it jointly.

A rough guide is:

Vi skal folde papkassen sammen

Something like:

vee skel FOL-duh PAP-kass-en SAM-men

A few useful notes:

• skal is often pronounced more like skel than a clear English skal
• folde has a soft second syllable
• papkassen has the stress at the beginning: PAP-
• sammen is usually pronounced with a light second syllable

If you want to sound natural, the rhythm matters a lot: the stressed syllables are mainly skal, fol-, pap-, and sam-.