Pasformen er behagelig, bortset fra at næsepuden er løs.

Pasform means fit—how well something sits or fits on the body or on an object.

Pasformen is the definite form, so it means the fit. Danish often adds the definite ending directly to the noun:

• pasform = fit
• pasformen = the fit

Here it refers to the specific fit of the item being discussed.

It can mean both, depending on context.

In this sentence, comfortable is the most natural English meaning, because it describes how the fit feels:

• Pasformen er behagelig = The fit is comfortable

In other contexts, behagelig can also mean pleasant, nice, or agreeable.

Because pasformen is a singular common-gender noun, and predicative adjectives usually stay in the base form with that kind of subject.

So:

• pasformen er behagelig
• næsepuden er løs

Compare:

• huset er behageligt = the house is comfortable/pleasant
  because huset is neuter
• skoene er behagelige = the shoes are comfortable
  because skoene is plural

Bortset fra at means except that or apart from the fact that.

So:

• bortset fra at næsepuden er løs = except that the nose pad is loose

It is a very natural way to mention one exception to an otherwise positive statement.

You can also use bortset fra without at if what follows is just a noun phrase:

• bortset fra næsepuden = except for the nose pad

Because after at, Danish uses normal subordinate-clause word order: subject + verb.

So:

• at næsepuden er løs

not:

• at er næsepuden løs

This is different from some main clauses in Danish, where the verb can come before the subject because of V2 word order. After at, that inversion does not happen.

Næsepuden means the nose pad.

It is a compound noun:

• næse = nose
• pude = pad / cushion
• næsepude = nose pad
• næsepuden = the nose pad

This is the kind of pad you would find on glasses, goggles, or a similar item.

Because næsepuden is singular and common gender, so the adjective takes the base form løs.

Compare:

• næsepuden er løs = the nose pad is loose
• båndet er løst = the ribbon/string is loose
  (båndet is neuter)
• snørebåndene er løse = the shoelaces are loose
  (plural)

So the form of the adjective changes to match gender and number.

Because there are really two clauses:

1. Pasformen er behagelig
2. næsepuden er løs

The second clause is introduced by bortset fra at. Danish, like English, still needs a full clause there.

So the structure is basically:

• The fit is comfortable
• except that the nose pad is loose

That is why er appears once in each clause.

Yes, absolutely. That would be natural Danish too.

The difference is mainly nuance:

• men = but
  more direct contrast
• bortset fra at = except that / apart from the fact that
  emphasizes that this is the one exception to an otherwise good situation

So bortset fra at sounds slightly more like a review or evaluation.

A rough guide:

• næse: the æ sounds somewhat like the vowel in bed, but longer
• næsepuden: roughly NEH-suh-poo-thən
• løs: the ø has no exact English equivalent; say something like luhs, but with rounded lips

A few extra notes:

• The d in puden is a soft Danish d, often a bit like the th in this
• The final -en in næsepuden is usually pronounced weakly

No. Pasform and størrelse are related, but they are not the same.

• pasform = fit, how something sits
• størrelse = size

So this sentence is not mainly talking about the item's size. It is talking about how the item feels when worn or used.