Hvis der kommer for meget støj fra gaden, lukker hun døren til altanen.

Hvis means if and introduces a condition. The first part, Hvis der kommer for meget støj fra gaden, sets up the situation under which the second part happens.

So the structure is:

• Hvis
  • condition
• main clause with the result

In English, this is the same basic pattern as If it gets too noisy from the street, she closes the balcony door.

Here der is a kind of dummy subject, similar to English there in sentences like There is a problem.

Danish often uses der when introducing something that exists or happens, especially before an indefinite noun phrase like for meget støj.

So:

• der kommer støj = literally something like there comes noise
• natural English: noise comes / there is noise coming

This der does not mean location here. It is grammatical, not spatial.

Danish often uses the present tense where English might also use the present tense in conditionals.

So in a sentence with if, Danish normally says:

• Hvis der kommer ... = If there comes / if there is ...
• not a special future form

This is very normal. Danish does not need a separate future tense here. The present tense can refer to a future possibility.

For meget means too much, while meget by itself means much / a lot of.

So:

• meget støj = a lot of noise
• for meget støj = too much noise

The word for adds the idea that the amount is excessive.

Støj is usually an uncountable noun, like noise in English. You normally treat it as a mass noun, not as individual countable items.

So Danish says:

• meget støj
• for meget støj

not usually a plural form in this kind of sentence.

This is because Danish main clauses follow the verb-second rule.

When the sentence starts with a subordinate clause like Hvis der kommer for meget støj fra gaden, that whole clause takes the first position. Then in the main clause, the finite verb must come next.

So the pattern becomes:

• Hvis der kommer for meget støj fra gaden, lukker hun døren til altanen.

Not:

• Hvis der kommer for meget støj fra gaden, hun lukker ...

This is one of the most important word-order rules in Danish.

Døren is the definite singular form of dør.

• en dør = a door
• døren = the door

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

Døren til altanen means the door to the balcony.

The preposition til is used because it indicates what the door leads to.

So:

• døren til altanen = the door to the balcony
• literally, the door leading to the balcony

This is a very common Danish pattern: noun + til + place.

Because it means the balcony, not just a balcony.

• en altan = a balcony
• altanen = the balcony

Again, Danish marks definiteness by adding an ending to the noun:

• -en for many common-gender nouns

Fra means from and shows the source or origin of the noise.

So:

• støj fra gaden = noise from the street

It tells you where the noise is coming from.

Yes, in normal Danish writing, a subordinate clause like the Hvis-clause is separated from the main clause with a comma.

So the comma marks the boundary between:

• Hvis der kommer for meget støj fra gaden and
• lukker hun døren til altanen

It helps show the sentence structure clearly.

Yes, you could. Altandøren means the balcony door.

So these are both possible:

• lukker hun døren til altanen
• lukker hun altandøren

The version in your sentence is slightly more explicit: it focuses on the door to the balcony. The compound noun altandøren is a bit more compact and is also very natural Danish.