Hagesmækken bliver hurtigt beskidt, når hun spiser grød og mos alene.

Because hagesmækken is the definite singular form: the bib.

The indefinite form is en hagesmæk = a bib.
Danish usually adds the definite article to the end of the noun:

• en hagesmæk = a bib
• hagesmækken = the bib

This is very common in Danish. The last part of the compound, smæk, is the part that determines the noun’s gender and inflection.

Bliver means becomes / gets, while er means is.

So:

• Hagesmækken er beskidt = The bib is dirty
• Hagesmækken bliver beskidt = The bib gets dirty / becomes dirty

In this sentence, the idea is that the bib ends up dirty as a result of the eating, so bliver is the natural choice.

Because hurtigt is an adverb, and it modifies the verb phrase bliver beskidt.

• hurtig = quick / fast as an adjective
• hurtigt = quickly as an adverb

So:

• en hurtig bil = a fast car
• hun spiser hurtigt = she eats quickly

In this sentence, hurtigt tells us how fast the bib gets dirty.

Because Danish often uses blive + adjective to express a change of state.

So:

• bliver beskidt = gets dirty
• bliver træt = gets tired
• bliver varm = gets warm

Here, beskidt is a predicative adjective. It describes the subject (hagesmækken) after the change happens.

That is the normal Danish word order here.

In bliver hurtigt beskidt, the adverb hurtigt comes before the adjective beskidt and modifies the whole change: gets dirty quickly.

If you moved hurtigt to the end, it would sound less neutral or more marked. The given version is the most natural everyday wording.

Når is used for when/whenever in a general or repeated situation.

Here, the sentence is not about one single event. It describes what usually happens:

• Hagesmækken bliver hurtigt beskidt, når hun spiser ...
• The bib gets dirty quickly when/whenever she eats ...

If this were about one specific past event, Danish might use something else depending on context, but når is right for a general pattern.

Because når introduces a subordinate clause, and subordinate clauses do not use normal Danish main-clause inversion.

Main clause:

• Hun spiser grød og mos.

Subordinate clause:

• når hun spiser grød og mos

In main clauses, Danish usually follows V2 word order, where the finite verb is in second position. But after a subordinating conjunction like når, the subject normally comes before the verb.

Because grød and mos are being used as mass nouns in a general sense.

So Danish says:

• spiser grød og mos = eats porridge and mash/purée

This is similar to English, where you also usually say eat porridge rather than eat a porridge.

If you were talking about a specific serving or a specific type, the structure could be different, but here it is just general food reference.

Here alene means alone / by herself / on her own.

In context, it most likely suggests that she is eating without help, which is why the bib gets dirty quickly. So the idea is not just that no one else is in the room, but that she is handling the eating herself.

That nuance is very common in Danish. In child-related contexts, alene often points to independent action.

Because Danish normally uses a comma before a subordinate clause.

So in:

• Hagesmækken bliver hurtigt beskidt, når hun spiser grød og mos alene.

the part starting with når is a subordinate clause, and the comma marks that boundary.

This is standard Danish punctuation, even if English punctuation may sometimes feel less strict in similar sentences.