Huset har ett larm och dess dörr är låst.

Because huset is the definite form of hus.

In Swedish, definiteness is often added as an ending:

• hus = house / a house
• huset = the house

So huset means that we are talking about a specific house, not just any house.

Because larm is an ett-word in Swedish.

Swedish nouns have two grammatical genders:

• en-words
• ett-words

Since larm belongs to the ett group, its indefinite article is ett:

• ett larm = an alarm

If it were definite, it would be larmet = the alarm.

Dess is the possessive word meaning its.

So dess dörr means its door.

A useful point: dess does not change depending on the noun after it. You use the same form with singular, plural, common gender, or neuter nouns.

For example:

• dess dörr = its door
• dess fönster = its window
• dess dörrar = its doors

Because after a possessive, Swedish normally uses the noun in its basic form, not the definite form.

So:

• dess dörr = its door
• min bok = my book
• hans bil = his car

The idea is that the possessive already makes the noun definite enough, so Swedish does not usually add the definite ending too.

That is why dess dörren is wrong.

This is a very common learner question.

Sin / sitt / sina are reflexive possessives. They refer back to the subject of the clause:

• Mannen låser sin dörr. = The man locks his own door.

But with an inanimate possessor like huset, Swedish often uses dess instead, especially in more careful or written language:

• Huset har ett larm och dess dörr är låst.

So here dess is the natural explicit equivalent of its.

That said, many Swedes would avoid dess in everyday speech unless they really need to be explicit.

Because låst is functioning like an adjective here, specifically a past participle from låsa = to lock.

So:

• är låst = is locked

This is a very common pattern in Swedish:

• Dörren är stängd. = The door is closed.
• Fönstret är öppet. = The window is open.
• Dörren är låst. = The door is locked.

With låst, the singular form is låst, while the plural is låsta:

• Dörren är låst. = The door is locked.
• Dörrarna är låsta. = The doors are locked.

Yes, it is grammatical, but it sounds a bit formal or bookish, mainly because of dess.

In everyday Swedish, people would often say:

• Huset har ett larm och dörren är låst.

If the context already makes it obvious which door is meant, that sounds more natural.

Another possible phrasing is:

• Huset har ett larm och husets dörr är låst.

That is also clear, though a little heavier in style.

So the given sentence is correct, but not the most conversational version.

The second part is a normal main clause:

• dess dörr = subject
• är = finite verb
• låst = predicative complement

So the pattern is:

subject + verb + complement

This is standard Swedish word order in a main clause.

Also, after och, the new clause still follows normal main-clause order.

Because Swedish usually marks definiteness with an ending, not with a separate word like English the.

Compare:

• English: the house
• Swedish: huset

This is one of the biggest structural differences between English and Swedish.

Swedish can also use a separate definite word in some situations, but with a simple noun like this, the normal form is just the noun plus the definite ending.

Yes, very often you could.

If the context already makes it obvious that the door belongs to the house, Swedish often prefers the simpler version:

• Huset har ett larm och dörren är låst.

That sounds more natural in everyday speech.

Using dess is mainly helpful when you want to be very explicit about the possessor.