Jeg nyser altid, når der er for meget støv i rummet.

Because nyser is the present tense of the verb at nyse (to sneeze). In Danish, the present tense is usually formed by adding -r to the infinitive.

• at nyse = to sneeze
• jeg nyser = I sneeze / I am sneezing

Also, Danish verbs do not change for different persons the way English verbs sometimes do. So jeg nyser, du nyser, vi nyser, etc.

In a normal Danish main clause, the finite verb usually comes in second position. Here, nyser is the finite verb, so it comes right after jeg, and altid follows it.

So the pattern is:

Jeg + nyser + altid

This is very common with adverbs like altid, ofte, aldrig, and tit. English often puts always before the main verb, but Danish usually puts altid after the finite verb in a simple sentence like this.

Here, når means when or whenever. It is used because the sentence describes something that happens regularly or repeatedly.

So this sentence means something like: whenever there is too much dust in the room, I sneeze.

Hvis means if, and it is more conditional or hypothetical. Compare:

• Når der er for meget støv i rummet, nyser jeg. = When/whenever there is too much dust in the room, I sneeze.
• Hvis der er for meget støv i rummet, nyser jeg måske. = If there is too much dust in the room, I might sneeze.

Der er means there is or there are. In this sentence, der is not really the location word there. It is a kind of dummy subject used in existential sentences.

So:

• der er støv = there is dust
• der er mange mennesker = there are many people

This is very similar to English there is/there are.

Because støv (dust) is an uncountable noun. With uncountable nouns in Danish, you use meget (much), not mange (many).

• for meget støv = too much dust
• for meget vand = too much water

But with countable plural nouns, you use mange:

• for mange bøger = too many books
• for mange sko = too many shoes

Also, for here means too, not for in the English sense.

Rummet is the definite form of et rum.

• et rum = a room
• rummet = the room

In Danish, the definite article is often added to the end of the noun as a suffix. Since rum is a neuter noun, the definite ending is -met, giving rummet.

So i rummet means in the room, not just in a room.

In Danish, a simple definite noun is usually formed with the ending on the noun itself:

• rummet = the room

You use det with the noun mainly when there is an adjective or when you want extra emphasis:

• det store rum = the big room
• det rum = that room / that particular room

So in this sentence, i rummet is the normal way to say in the room.

Because når der er for meget støv i rummet is a subordinate clause, and in standard Danish writing, a comma is placed before a subordinate clause when it comes after the main clause.

So the sentence is divided like this:

• main clause: Jeg nyser altid
• subordinate clause: når der er for meget støv i rummet

That is why the comma appears before når.

Yes. If the subordinate clause comes first, the main clause still follows the Danish verb-second rule, so the finite verb comes before the subject in the main clause.

So you get:

Når der er for meget støv i rummet, nyser jeg altid.

Notice that it is nyser jeg, not jeg nyser. This inversion is very common in Danish when something other than the subject comes first.

Because the sentence describes a general habit or something that happens regularly. Danish often uses the present tense for this, just like English does.

So:

• Jeg nyser altid = I always sneeze
• når der er for meget støv = when there is too much dust

It is not talking about one specific moment, but about a repeated situation.