Hún var standandi við gluggann þegar ég kom inn með kaffið.

In Icelandic, the definite article is usually attached to the end of the noun instead of appearing as a separate word.

• gluggi = window
• gluggann = the window (here, accusative singular)
• kaffi = coffee
• kaffið = the coffee

So English the window and the coffee are built directly into the noun forms in Icelandic.

Both can work, but they are not exactly the same in tone.

• Hún stóð við gluggann = She stood / was standing by the window
• Hún var standandi við gluggann = more explicitly She was standing by the window

Using vera + present participle (var standandi) can make the scene feel more descriptive or ongoing. It highlights her state at the moment when something else happened.

A more neutral, everyday version would very often be:

• Hún stóð við gluggann þegar ég kom inn með kaffið.

So the sentence as given is grammatical and understandable, but slightly more marked in style.

Standandi is the present participle of standa (to stand).

It is formed with -andi, a very common participle ending in Icelandic:

• standa → standandi
• hlæja → hlæjandi (laughing)
• syngja → syngjandi (singing)

In this sentence, standandi is used with var to describe an ongoing posture or state:

• var standandi = was standing

English uses was standing very naturally; Icelandic can do something similar, but it does not rely on this kind of progressive form as much as English does.

Við gluggann means by the window or at the window.

Here:

• við = by, at, next to
• gluggann = the window in the form required here

The noun gluggi changes form because Icelandic nouns change for case. In this expression, við takes the accusative, so you get:

• gluggi → gluggann

So the whole phrase means that she was positioned next to the window, not literally inside it.

Because þegar introduces a subordinate clause, and subordinate clauses do not follow the same word-order pattern as main clauses.

Main clauses in Icelandic usually follow a verb-second pattern, but after a subordinating word like þegar (when), the subject normally comes before the verb:

• þegar ég kom inn = when I came in

So this is the normal order:

• þegar
  • subject
    • verb

That is why ég comes before kom here.

Inn means in / inside, and with a motion verb like koma it shows movement into a place.

• koma = come
• koma inn = come in

So:

• ég kom inn = I came in

This is very common in Icelandic: a basic verb is often combined with a small directional word like inn, út, upp, or niður to make the meaning more specific.

This is a very good question, because learners often expect með to be followed by the dative.

If með were just the ordinary preposition with, then yes, you would normally expect:

• með kaffinu = with the coffee

But in a sentence like this, koma inn með kaffið is best understood as a verb phrase meaning something like bring the coffee in or come in carrying the coffee. In other words, kaffið functions as the object of the action, not simply as the object of a plain preposition.

So:

• ég kom inn með kaffið = I came in with the coffee / I brought the coffee in

That is why the accusative kaffið makes sense here.

The sentence presents one action as the background and another as the event that happens during it.

• Hún var standandi við gluggann = background situation already in progress
• þegar ég kom inn með kaffið = the event that happens at that time

So the meaning is that she was already standing by the window at the moment I came in with the coffee.

This is very similar to English:

• She was standing by the window when I came in with the coffee.

It is understandable and grammatical, but many native speakers would probably choose the simpler verb stóð in everyday speech:

• Hún stóð við gluggann þegar ég kom inn með kaffið.

That version is very natural and probably more common.

The original version with var standandi sounds a bit more descriptive, scene-setting, or stylistically marked. So it is not wrong; it just has a slightly different feel.