Min søn samler papkasser, fordi han vil bygge et lille hus af pap til kaninen.

After a possessive like min, Danish normally uses the noun in its basic form, not the definite form.

So:

• min søn = my son
• sønnen = the son

You usually do not combine min with the definite ending -en here.

Samler is the present tense of samle.

A very common pattern in Danish is:

• infinitive: samle
• present tense: samler

So Min søn samler papkasser means that he collects / is collecting cardboard boxes.

Danish makes compound nouns very freely, much more like German than English.

So:

• pap = cardboard
• kasse = box
• papkasse = cardboard box
• papkasser = cardboard boxes

In English we write cardboard box as two words, but in Danish it is normally one word.

Because papkasser is an indefinite plural noun here.

Danish often uses a bare plural where English also does:

• Han samler papkasser = He collects cardboard boxes

If it were definite, it would be:

• papkasserne = the cardboard boxes

Because fordi introduces a subordinate clause: because he wants to build...

In standard Danish writing, a comma is normally placed before a subordinate clause like this.

So the sentence is divided into:

• main clause: Min søn samler papkasser
• subordinate clause: fordi han vil bygge et lille hus af pap til kaninen

Because after fordi, Danish uses subordinate clause word order.

In a main clause, Danish usually has verb-second order:

• Han vil bygge...

But in a subordinate clause introduced by fordi, the subject normally comes before the finite verb:

• fordi han vil bygge...

That is the normal pattern.

Because vil is a modal verb, and after modal verbs Danish uses the bare infinitive.

So:

• han vil bygge = he wants to build / he will build

Not:

• han vil at bygge

This is similar to English:

• he wants to build
• but with modals: he will build, not he will to build

It can mean either, depending on context, but in this sentence it most naturally means wants to or intends to.

So han vil bygge et lille hus is best understood as:

• he wants to build a small house

Danish vil often overlaps with English will, wants to, or intends to, so context matters.

Because hus is a neuter noun, so it takes et in the indefinite singular.

Danish nouns are mainly either:

• en-words
• et-words

And hus is an et-word:

• et hus = a house

So:

• et lille hus = a small house

Because lille is irregular.

Many adjectives add -t with a singular neuter noun:

• et stort hus = a big house

But lille stays lille:

• et lille hus
• en lille bil

So lille is just a form you need to learn as a special case.

Af is used to show what something is made of.

So:

• et hus af pap = a house made of cardboard

This is the natural choice for material.

By contrast, fra usually means from in the sense of origin or source, not material composition.

Til often means for in the sense of intended for or meant for.

So:

• et lille hus af pap til kaninen = a small cardboard house for the rabbit

You can think of til here as showing the destination or intended user of the house.

Because kaninen is the definite form: the rabbit.

In Danish, definiteness is often shown by an ending:

• en kanin = a rabbit
• kaninen = the rabbit

So the sentence refers to a specific rabbit, probably one already known in the situation.

The ø sound does not exist exactly in English, which is why many learners notice it immediately.

A rough guide:

• ø is somewhat like the vowel in British English bird, but without the r
• søn is roughly like sun said with rounded lips, though that is only an approximation

The important point is that ø is its own vowel in Danish, and you should not pronounce it like English o.