Når vi pendler om morgenen, er der mere støj i toget, end jeg kan lide.

Når is used for something that happens repeatedly or whenever it happens.

So Når vi pendler om morgenen means something like When/Whenever we commute in the morning.

By contrast, da is usually used for a specific past occasion, for example Da vi pendlede i morges... = When we were commuting this morning... / When we commuted this morning...

So in this sentence, når fits because it describes a general habit, not one single past event.

Pendler is the present tense of at pendle, which means to commute or to travel back and forth regularly, especially between home and work or school.

So vi pendler = we commute.

It is more specific than just travel. It suggests a regular routine.

Danish does not have a separate tense like the English present continuous.

English often distinguishes between:

• we commute
• we are commuting

Danish usually just uses the present tense: vi pendler.

The time meaning comes from context. In this sentence, the whole clause sounds habitual, so vi pendler naturally means when we commute or when we are commuting, depending on how you express it in English.

Om morgenen is a very common Danish time expression meaning in the morning or, in a habitual sentence like this, often in the mornings.

A few useful patterns are:

• om morgenen = in the morning
• om aftenen = in the evening
• om natten = at night

Here, om is the usual preposition for this type of general time expression.

In Danish, many time expressions use the definite form:

• om morgenen
• om aftenen
• om dagen

So even though English says in the morning, where the is a separate word, Danish often builds the definiteness into the noun itself: morgen → morgenen.

This is just the normal idiomatic form in Danish.

Because Danish main clauses normally follow the V2 rule: the finite verb comes in the second position.

Here, the whole first clause Når vi pendler om morgenen takes the first position. That means the finite verb of the main clause must come next:

• Når vi pendler om morgenen, er der mere støj i toget ...

So the order is:

1. Når vi pendler om morgenen
2. er
3. der
4. mere støj i toget

If the sentence started directly with the main clause, then you would get:

• Der er mere støj i toget ...

Both are correct; the word order changes because the sentence begins with a subordinate clause.

This der is the Danish equivalent of the dummy there in English expressions like:

• There is more noise
• There are many people

So der er means there is / there are.

It does not mean there as a place in this sentence. It is just required by the existential construction.

Because støj means noise, and it is an uncountable noun.

In Danish:

• mere = more, used with uncountable nouns
• flere = more, used with countable plural nouns

So:

• mere støj = more noise
• flere mennesker = more people

Since you cannot count noise as separate items in this sentence, mere is the correct choice.

Again, because støj is usually treated as an uncountable noun.

Just as English often says there is noise or more noise without a, Danish says:

• der er støj
• der er mere støj

You would not normally use an indefinite article with støj in this kind of sentence.

This is a very common difference between Danish and English.

Danish often uses i where English uses on for public transport:

• i toget = on the train
• i bussen = on the bus

Literally, i toget means in the train, but idiomatic English normally says on the train. So this is something you usually just learn as a Danish-English difference.

Toget is the definite form of tog:

• et tog = a train
• toget = the train

In this sentence, Danish uses the definite form because it refers to the train involved in the commuting situation: the train we are on.

Here, end means than.

It is used after a comparative:

• mere ... end ... = more ... than ...
• større end = bigger than
• bedre end = better than

So:

• mere støj ... end jeg kan lide
  means
• more noise ... than I like

Even though end looks like the English word end, it has a completely different meaning here.

Yes, historically and word-for-word it looks like can like, but in modern Danish kan lide is a normal fixed expression meaning like.

So:

• jeg kan lide kaffe = I like coffee
• jeg kan lide at læse = I like reading / I like to read

In this sentence, mere støj, end jeg kan lide means more noise than I like or more noise than I care for.

So you should usually learn kan lide as one expression.

Because end introduces a comparison clause, and the word order is not main-clause inversion.

So you get normal subject + verb order:

• end jeg kan lide

Compare:

• main clause: Jeg kan lide det
• comparison clause: end jeg kan lide

There is no reason to invert here.

Because Når vi pendler om morgenen is a subordinate clause, and it is followed by the main clause.

In standard Danish, a comma is normally placed between a subordinate clause and the main clause:

• Når vi pendler om morgenen, er der mere støj i toget ...

So the comma is marking the boundary between the two parts of the sentence.