I og med at kjolen er lang, legges den opp.

I og med at is a conjunction meaning since, as, given that, or because of the fact that.

In this sentence, it introduces the reason:

• I og med at kjolen er lang = Since the dress is long / Given that the dress is long

It is a slightly more formal or explanatory expression than just fordi or siden.

Because i og med at is a fixed expression. You should learn it as one whole conjunction.

So:

• i og med at ... = since / given that ...

You normally would not break it up or replace at with something else here.

This is because of Norwegian V2 word order in main clauses.

The sentence begins with a subordinate clause:

• I og med at kjolen er lang

When that clause comes first, the finite verb in the main clause must come second:

• legges den opp

So the structure is:

1. Fronted subordinate clause
2. Finite verb
3. Subject

Compare:

• Den legges opp. = It is hemmed.
• I og med at kjolen er lang, legges den opp.

This is very common in Norwegian.

Here legges opp means is hemmed up or is shortened at the hem.

The verb expression is legge opp, which is often used about clothes such as trousers, skirts, and dresses. It means adjusting the length by folding and sewing the bottom.

So in context:

• kjolen legges opp = the dress is hemmed / the dress is taken up

Legges is the -s passive form of legge.

Active:

• Man legger opp kjolen. = Someone hems the dress.

Passive:

• Kjolen legges opp. = The dress is hemmed.

Norwegian often uses the passive when the person doing the action is not important or not mentioned. In this sentence, the focus is on what happens to the dress, not on who does it.

Den refers to kjolen (the dress).

Since kjole is a common-gender noun, the pronoun is den:

• kjolen → den

So:

• legges den opp = it is hemmed up

Kjolen is the definite form, meaning the dress.

• en kjole = a dress
• kjolen = the dress

Using the definite form suggests we are talking about a specific dress already known from the context.

Yes. That would be natural too.

Possible alternatives include:

• Siden kjolen er lang, legges den opp.
• Fordi kjolen er lang, legges den opp.
• Ettersom kjolen er lang, legges den opp.

The main difference is style and nuance:

• i og med at = somewhat formal, explanatory
• siden = natural and common for since
• fordi = straightforward because

Literally, lang means long. But in this context, it strongly implies too long or longer than desired.

That is understood from the result:

• if the dress legges opp, it is being shortened

So the sentence naturally means something like:

• Since the dress is too long, it is hemmed up.

Yes, that is a good idea. Legge opp is a verb phrase, and in many sentences its parts can be separated.

For example:

• De legger opp kjolen.
• Kjolen legges opp.

This kind of split is very common with Norwegian verb + particle combinations. So it is useful to learn legge opp as a set expression meaning to hem / take up clothing.