Min datters madkasse står stadig i bilen, så jeg må gå tilbage og hente den.

Because datters is the genitive form, meaning daughter’s.

In Danish, possession is often shown by adding -s to the noun:

• min datter = my daughter
• min datters madkasse = my daughter’s lunchbox

This works a lot like English ’s, but Danish usually does not use an apostrophe.

So:

• datter = daughter
• datters = daughter’s

Because the possessive already makes it specific.

In English, we say my daughter’s lunchbox, not my daughter’s a lunchbox or the my daughter’s lunchbox. Danish works the same way.

So:

• en madkasse = a lunchbox
• madkassen = the lunchbox
• min datters madkasse = my daughter’s lunchbox

When a noun has a possessor like min datters, you do not also add an article.

Madkasse literally means food box, but in everyday English it is usually lunchbox.

It is made up of:

• mad = food
• kasse = box

So madkasse is a very natural Danish word for the container someone takes food in, especially for school or work.

Danish often prefers a position verb where English would just say is.

Here, står literally means stands, but it is commonly used for objects that are upright or thought of as being placed somewhere.

So:

• Madkassen står i bilen = The lunchbox is in the car

Even though English would usually not say stands, Danish often does.

Common Danish position verbs include:

• stå = stand
• ligge = lie
• sidde = sit

Examples:

• Flasken står på bordet = The bottle is on the table
• Bogen ligger på sengen = The book is on the bed
• Nøglen sidder i låsen = The key is in the lock

So står sounds more natural than a generic er here.

Stadig means still.

So:

• står stadig i bilen = is still in the car

Its placement is normal for a Danish main clause. Danish usually has the finite verb early in the sentence, and adverbs like stadig often come after it:

• Madkassen står stadig i bilen

Compare:

• Den er stadig her = It is still here

So the position of stadig is not random; it follows normal Danish word order.

Because bilen means the car, referring to a specific car already understood from context.

• i en bil = in a car
• i bilen = in the car

In this sentence, the speaker means a particular car, probably their own or the one both speaker and listener know about. That is why Danish uses the definite form bilen.

Also remember that Danish usually adds the definite article to the end of the noun:

• en bil = a car
• bilen = the car

Here så means so.

It connects the two ideas:

• Min datters madkasse står stadig i bilen
• så jeg må gå tilbage og hente den

In other words:

My daughter’s lunchbox is still in the car, so I have to go back and get it.

This is a very common use of så in everyday Danish.

Må often means must, have to, or sometimes am allowed to, depending on context.

Here it clearly means have to / must:

• jeg må gå tilbage = I have to go back

That is because the situation creates a necessity: the lunchbox is in the car, so the speaker needs to go get it.

Compare:

• Du må gerne gå = You may go / You’re allowed to go
• Jeg må skynde mig = I have to hurry

So må can express either obligation or permission, and context tells you which one is meant.

Because after a modal verb like må, Danish uses the bare infinitive.

So:

• jeg må gå
• jeg må hente
• not jeg må at gå

This is similar to English:

• I must go
• not I must to go

In the sentence, there are two infinitives linked by og:

• gå tilbage = go back
• hente den = get it / fetch it

So:

• jeg må gå tilbage og hente den = I have to go back and get it

Tilbage means back.

So:

• gå tilbage = go back

It often refers to returning to a previous place.

Examples:

• Jeg går tilbage til bilen = I’m going back to the car
• Han kom tilbage i går = He came back yesterday

In this sentence, the speaker has moved away from the car and now needs to return to it.

Den means it, and it refers back to madkasse.

Danish nouns have grammatical gender, and pronouns must match that gender:

• en madkasse → den
• et hus → det

Since madkasse is a common-gender noun (en madkasse), the correct pronoun is den.

So:

• hente den = get it

If the noun were a neuter noun, Danish would use det instead.

No, it is normal main-clause word order.

Danish is a V2 language, which means the finite verb usually comes in the second position in a main clause. But here the second clause begins with så, which is a coordinating conjunction, so the clause itself still has normal order:

• jeg = subject
• må = finite verb

So:

• så jeg må gå tilbage og hente den

This is similar to English so I have to go back and get it.

If something else came first in the clause, the verb would still need to stay in second position.

Yes:

• Min = my
• datters = daughter’s
• madkasse = lunchbox
• står = stands / is
• stadig = still
• i = in
• bilen = the car
• så = so
• jeg = I
• må = must / have to
• gå = go
• tilbage = back
• og = and
• hente = get / fetch
• den = it

A very natural English rendering is:

My daughter’s lunchbox is still in the car, so I have to go back and get it.

Yes, this sounds natural and idiomatic.

A few especially natural features are:

• datters for possession
• står for an object being somewhere
• stadig for still
• må for have to
• den referring back to madkasse

A speaker might also say something slightly different depending on style, for example:

• Min datters madkasse er stadig i bilen...
• Jeg bliver nødt til at gå tilbage og hente den.

But the original sentence is completely natural Danish.