Du kan stille kassen hvor som helst i gangen, men ikke foran døren.

In this sentence, stille means to place / put / set something somewhere.

A useful rough distinction is:

• stille = place something, often thought of as standing or being set in position
• sætte = set/put, especially for many everyday actions
• lægge = lay something down

With kassen (the box), stille sounds very natural in Danish. It suggests placing the box somewhere in the hallway. In everyday speech, sætte may also be possible in some contexts, but lægge would more strongly suggest laying the box on its side or flat.

That -en is the usual Danish way of marking the definite form of many common-gender nouns.

So:

• en kasse = a box

• kassen = the box

• en gang = a hallway / corridor

• gangen = the hallway

• en dør = a door
• døren = the door

Unlike English, Danish usually adds the definite article to the end of the noun instead of using a separate word like the.

Because after a modal verb like kan (can), Danish uses the bare infinitive.

So you get:

• du kan stille
• jeg vil gå
• vi skal vente

not:

• du kan at stille

This is similar to English, where we say you can put it, not you can to put it.

Hvor som helst means anywhere.

Literally, it is built from:

• hvor = where
• som helst = something like at all / whatever / you like, depending on context

Together, hvor som helst functions as a fixed expression meaning anywhere.

Examples:

• Du må sidde hvor som helst. = You may sit anywhere.
• Jeg kan bo hvor som helst. = I can live anywhere.

So hvor som helst i gangen means anywhere in the hallway.

Here i means in because the box can be placed somewhere within the space of the hallway.

So:

• i gangen = in the hallway

This is the most natural choice when talking about location inside that area. Danish often uses i for enclosed or clearly defined spaces.

You may sometimes hear på gangen in certain contexts, especially in institutions or regionally, but i gangen is very normal and straightforward here.

Because Danish, like English, often leaves out repeated words when they are easy to understand from context.

The full version could be:

• Du kan stille kassen hvor som helst i gangen, men du kan ikke stille kassen foran døren.

But that sounds heavier and more repetitive.

So Danish shortens it to:

• men ikke foran døren

This is a very common kind of ellipsis. English does the same:

• You can put the box anywhere in the hallway, but not in front of the door.

In the shortened second part, ikke directly negates the location phrase:

• ikke foran døren = not in front of the door

Because the verb is omitted, ikke is placed right before the part being denied.

If you wrote the second part as a full clause, the word order would change:

• ... men du kan ikke stille kassen foran døren.

There, ikke comes after the finite verb kan, which is the normal position in a main clause.

A fully expanded version would be:

• Du kan stille kassen hvor som helst i gangen, men du kan ikke stille den foran døren.

or:

• ... men du kan ikke stille kassen foran døren.

Using den is especially natural because it avoids repeating kassen.

So the sentence you were given is simply a shorter, more natural version of that.

The first clause follows normal Danish main-clause word order:

• Du = subject
• kan = finite verb
• stille = infinitive
• kassen = object
• hvor som helst i gangen = place expression

So the structure is basically:

subject + finite verb + infinitive + object + location

That is very standard after a modal verb like kan.

Not in this sentence. Here foran means in front of.

So:

• foran døren = in front of the door

Even though English before and Danish foran may look a bit related historically, you should not translate foran as before in normal modern usage when talking about space.

Examples:

• Bilen står foran huset. = The car is in front of the house.
• Han står foran mig. = He is standing in front of me.

Grammatically, du is singular you. In this sentence it is most naturally addressing one person directly.

But in practice, Danish can also use du in a fairly general instructional way, much like English you:

• Du må ikke parkere her. = You must not park here.

Modern Danish normally uses du as the standard way to address someone. The old formal De exists but is much less common.