Skisekken med sitteunderlaget er lett.

Because Norwegian normally writes compound nouns as a single word.

• ski
  • sekk = skisekk
• with the definite ending: skisekken

This is very common in Norwegian. English often uses two words where Norwegian uses one, so this is something learners notice a lot.

The ending -en marks the noun as definite: the ski backpack.

• en skisekk = a ski backpack
• skisekken = the ski backpack

So Norwegian usually puts definiteness on the end of the noun instead of using a separate word like English the.

Because in Norwegian, definiteness is usually built into the noun itself.

Here you see that in:

• skisekken = the ski backpack
• sitteunderlaget = the seat pad / sitting mat

This is one of the biggest differences from English. English says the backpack; Norwegian often says backpack-the, in effect.

It is another compound noun.

• sitte = related to sitting
• underlag = base, surface, pad, layer

Together, sitteunderlag means something like a sitting pad or seat pad. In outdoor Norwegian, this is a very common kind of word: a practical object name made by combining smaller words into one compound.

Because underlag is a neuter noun, usually an et-word.

• et sitteunderlag = a seat pad
• sitteunderlaget = the seat pad

So:

• -en often marks definite singular for en-words
• -et often marks definite singular for et-words

It comes from the verb å sitte = to sit, but in compounds it works more like a descriptive element.

So sitteunderlag means an underlay/pad for sitting on. Norwegian does this a lot:

• drikkeflaske = drinking bottle
• soverom = sleeping room / bedroom
• skrivebord = writing table / desk

So yes, it is related to a verb, but inside the compound it functions as part of a noun.

Med means with.

In this sentence, med sitteunderlaget is a phrase that describes the backpack more specifically: it is the ski backpack with the seat pad.

So med introduces extra information attached to the noun.

Yes. The whole subject is:

Skisekken med sitteunderlaget

Inside that subject:

• skisekken is the main noun
• med sitteunderlaget is a prepositional phrase that modifies it

Then the rest of the sentence is:

• er = the verb
• lett = the description/predicate adjective

Because this is normal Norwegian main-clause word order.

The sentence structure is:

• Skisekken med sitteunderlaget = subject
• er = verb
• lett = complement

Norwegian main clauses follow the verb-second pattern, often called V2. Here the subject comes first, so the verb naturally comes next.

Lett is a predicate adjective after er, just like English light in The backpack is light.

Adjectives in Norwegian do agree, but with lett the singular forms happen to look the same:

• sekken er lett
• underlaget er lett

So even though sekk and underlag have different genders, lett still looks the same here.

A clearer comparison is with another adjective:

• sekken er stor
• underlaget er stort

That shows gender agreement more visibly.

Yes. Med is fairly broad.

Depending on context, Skisekken med sitteunderlaget could mean:

• the backpack that comes with the seat pad
• the backpack that has the seat pad in it
• the backpack associated with that seat pad
• the backpack with the built-in/attached pad

The exact nuance depends on the real-world situation, not just the grammar.

A possible indefinite version would be:

En skisekk med et sitteunderlag er lett.

That means a ski backpack with a seat pad is light.

This helps show the pattern:

• en skisekk → skisekken
• et sitteunderlag → sitteunderlaget