På natbordet står en lille sengelampe, som jeg tænder, når jeg læser om aftenen.

Because Danish main clauses follow the verb-second rule.

In this sentence, På natbordet has been moved to the front for emphasis. When something other than the subject comes first, the finite verb must still stay in second position, so the subject moves after the verb:

• På natbordet står en lille sengelampe.

A more neutral order would be:

• En lille sengelampe står på natbordet.

Both are grammatical, but the original sentence highlights the location first.

Danish often uses so-called position verbs where English would often just say is.

Common ones are:

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

A lamp placed upright on a bedside table is naturally thought of as something that stands, so står sounds very natural in Danish.

So Danish prefers:

• Lampen står på natbordet.

where English would usually say:

• The lamp is on the bedside table.

Using er is not always impossible, but står is more idiomatic and more specific.

The ending -et is the definite article attached to the noun.

Danish usually puts the at the end of the noun instead of using a separate word.

• et natbord = a bedside table
• natbordet = the bedside table

Here, natbord is a neuter noun, which is why the definite ending is -et.

Compare:

• en lampe → lampen
• et bord → bordet

Because på means on, and a lamp is normally on top of a bedside table, not inside it.

• på natbordet = on the bedside table
• i natbordet would mean inside the bedside table, which would only make sense in a very unusual situation

So this is mainly about the real-world location of the lamp.

This is about how identifiable each thing is in the context.

• natbordet = the bedside table
  This sounds like a specific, known table.
• en lille sengelampe = a small bedside lamp
  This introduces the lamp as a piece of information.

That is very natural in Danish. A sentence often presents:

• a known location or setting first
• then a new thing within that setting

So the structure is roughly:

• On the bedside table → known place
• stands a small bedside lamp → new item being introduced

Sengelampe is a common-gender noun, so the indefinite article is en:

• en sengelampe = a bedside lamp

The adjective lille means small and goes before the noun:

• en lille sengelampe = a small bedside lamp

A useful detail: lille is a bit special and often stays lille in forms where other adjectives would change.

Compare:

• en stor lampe = a big lamp
• et stort bord = a big table

But:

• en lille lampe
• et lille bord

So lille is the correct form here.

Because Danish, like German, very often forms compound nouns as a single word.

• seng = bed
• lampe = lamp
• sengelampe = bed-lamp = bedside lamp

This is completely normal in Danish. The last part usually determines the basic meaning and the grammatical gender.

So:

• sengelampe is a kind of lampe
• therefore it takes the article of lampe: en sengelampe

Som is a relative pronoun here. It introduces a clause that describes sengelampe:

• en lille sengelampe, som jeg tænder
  = a small bedside lamp that I turn on

So som is like English that or which in a relative clause.

It links the extra information back to the lamp.

Because in this clause, the relative word refers to the object, not the subject.

In:

• som jeg tænder

the subject is jeg, and the thing being turned on is the lamp. So the relative word stands for the object, and som is the normal choice.

Compare:

• en lampe, der står på bordet
  = a lamp that is standing on the table
  Here, the lamp is the subject of står, so der is natural.

• en lampe, som jeg tænder
  = a lamp that I turn on
  Here, the lamp is the object of tænder, so som is used.

Because the sentence describes a habit or a general routine.

• jeg tænder = I turn on / I switch on

Together with når jeg læser om aftenen, it means this is something the speaker regularly does when reading in the evening.

Also, notice that the object is not repeated:

• som jeg tænder

The object is already understood from som, which refers back to sengelampe.

Når is used for when in the sense of something that happens repeatedly or whenever.

So:

• når jeg læser om aftenen
  means when/whenever I read in the evening

This fits a habitual action.

Important contrast:

• når = when, whenever, in general or in the future
• da = when, on one specific occasion in the past
• hvis = if

So here når is correct because the sentence describes a regular pattern, not a single past event.

Because læse can be used without an object when the meaning is simply read in a general sense.

So:

• jeg læser = I read / I am reading

Danish does not need to say a book, a magazine, and so on if that information is not important.

If you wanted to be more specific, you could say:

• når jeg læser en bog om aftenen
  = when I read a book in the evening

But in the original sentence, the exact thing being read does not matter.

Om aftenen means in the evening or in the evenings, often with a general or habitual sense.

So:

• når jeg læser om aftenen
  = when I read in the evening / in the evenings

This fits the idea of a routine.

By contrast:

• i aften = tonight

So these are different:

• Jeg læser om aftenen. = I read in the evenings.
• Jeg læser i aften. = I am reading tonight.

That difference is very important in Danish time expressions.

The commas mark subordinate clauses.

• som jeg tænder is a relative clause
• når jeg læser om aftenen is a subordinate time clause inside that larger structure

So the commas help show the sentence structure.

You should also know that Danish has had more than one accepted comma style, so you may sometimes see fewer commas in modern writing. But the commas in this sentence are perfectly normal and clearly mark the clauses.