Aftenrutinen bliver lettere, når børnene går tidligt i seng.

Because Danish forms compound nouns very freely.
Here, aften + rutine becomes aftenrutine = evening routine.

Then Danish adds the definite ending directly to the noun, so:

• aftenrutine = an evening routine
• aftenrutinen = the evening routine

This is very common in Danish. English often writes similar ideas as separate words, but Danish usually combines them.

That -n is the definite article, meaning the.

In Danish, the is often attached to the end of the noun instead of standing as a separate word in front of it.

For this noun:

• en rutine = a routine
• rutinen = the routine

Since aftenrutine ends in an unstressed -e, the definite singular form is aftenrutinen.

So English the evening routine becomes Danish aftenrutinen.

Bliver means becomes / gets, while er means is.

So:

• Aftenrutinen bliver lettere = The evening routine becomes / gets easier
• Aftenrutinen er lettere = The evening routine is easier

Using bliver suggests a change: when the children go to bed early, the routine turns out easier than it otherwise would be.

In natural English, this is often best understood as gets easier.

Lettere is the comparative form of let.

• let = easy / light
• lettere = easier
• lettest = easiest

So Danish often forms comparatives by adding -ere, much like English sometimes adds -er.

Here, lettere means easier.

Når usually means when in the sense of something that happens regularly, generally, or is expected to happen.

Hvis means if.

So:

• når børnene går tidligt i seng = when the children go to bed early
• hvis børnene går tidligt i seng = if the children go to bed early

In this sentence, the idea is that this is a typical pattern or general truth: whenever that happens, the evening routine gets easier. That is why når fits better.

Børnene means the children, while børn means children in a more general or indefinite way.

Here the sentence is talking about specific, understood children, so Danish uses the definite plural:

• barn = child
• børn = children
• børnene = the children

Notice that barn has an irregular plural: it does not become barner or anything like that. The plural is børn.

Gå i seng is a fixed expression meaning go to bed.

Literally, gå means go / walk, and i seng means to bed / into bed, but as a whole expression it simply means go to bed.

So:

• børnene går tidligt i seng = the children go to bed early

It does not necessarily emphasize physical walking. It is just the normal Danish phrase.

Because tidligt is being used adverbially here.

• tidlig = early as an adjective
• tidligt = early as an adverb

In this sentence, it describes when they go to bed, so it functions like an adverb:

• går tidligt i seng = go to bed early

A useful pattern is that Danish often uses the -t form as an adverb.

Compare:

• en tidlig morgen = an early morning
• han kom tidligt = he arrived early

After når, Danish introduces a subordinate clause.

In this clause, the order is straightforward:

• når = when
• børnene = the children
• går = go
• tidligt = early
• i seng = to bed

So the clause is:

når børnene går tidligt i seng

If you put that clause first in the sentence, the main clause changes word order because Danish main clauses follow the V2 rule:

• Når børnene går tidligt i seng, bliver aftenrutinen lettere.

Notice that bliver comes before aftenrutinen there. That inversion is normal in Danish main clauses after an opening subordinate clause.

Because når børnene går tidligt i seng is a subordinate clause.

In Danish, many writers place a comma before a subordinate clause like this. You may also see Danish written without that starting comma, because both comma systems are accepted in many contexts.

So you may encounter both:

• Aftenrutinen bliver lettere, når børnene går tidligt i seng.
• Aftenrutinen bliver lettere når børnene går tidligt i seng.

The version with the comma is very common and especially helpful for learners because it clearly shows where the subordinate clause begins.