Við horfum út um gluggann þegar það rignir.

Because horfum is the present-tense form that matches við (we).

The verb is að horfa (to look / watch). In the present tense:

• ég horfi — I look
• þú horfir — you look
• hann/hún/það horfir — he/she/it looks
• við horfum — we look
• þið horfið — you all look
• þeir/þær/þau horfa — they look

So Við horfum simply means we look.

Út um gluggann means out through the window or, more naturally in English, out the window.

It is made up of:

• út — out
• um — through / via / around, depending on context
• gluggann — the window

In this sentence, út um works together as a phrase meaning out through. So the idea is not that you are looking at the window, but that your gaze goes out through it.

Gluggann is the accusative singular definite form of gluggi (window).

Here is the basic pattern:

• gluggi — a window
• glugginn — the window (nominative)
• gluggann — the window (accusative)
• glugganum — the window (dative)

It is gluggann here because the preposition um takes the accusative in this expression: út um gluggann.

So:

• um + accusative → gluggann

Also, the -ann ending includes the definite article, so gluggann means the window, not just window.

That is a very good question, because horfa á is extremely common.

• að horfa á eitthvað = to look at something / watch something
  
  • Við horfum á sjónvarpið. — We watch the television.
  • Hún horfir á mig. — She looks at me.

But in your sentence, the structure is different:

• horfa út um gluggann = to look out the window

Here the focus is not on an object being watched, but on the direction your gaze goes. So Icelandic uses út um rather than á.

Þegar means when.

It introduces a time clause:

• þegar það rignir — when it rains

So the whole sentence means:

• Við horfum út um gluggann þegar það rignir.
• We look out the window when it rains.

Here það is a dummy subject, just like English it in it rains.

It does not refer to a real thing. It is simply required by the grammar of the sentence.

So:

• það rignir — it rains
• það snjóar — it snows

This is very similar to English, where it in it is raining does not point to any specific object.

Because Icelandic often uses the simple present for habitual or repeated actions, just like English can.

So Við horfum út um gluggann þegar það rignir can mean:

• We look out the window when it rains
• We look out through the window whenever it rains

It describes a general habit or repeated situation, not necessarily something happening only right now.

Depending on context, the Icelandic present tense can also sometimes sound more immediate, but in this sentence the habitual meaning is very natural.

The sentence has a very normal Icelandic word order:

• Við — subject
• horfum — finite verb
• út um gluggann — adverbial/prepositional phrase
• þegar það rignir — subordinate clause

So the structure is basically:

• We look out the window when it rains.

Nothing unusual is happening in the main clause.

Inside the subordinate clause, þegar það rignir, the order is also straightforward:

• það — subject
• rignir — verb

Yes. In natural English, out the window is usually the best translation.

Even though út um gluggann is more literally out through the window, English normally says:

• look out the window

rather than

• look out through the window

Both are possible in English, but the shorter version is more idiomatic.

It is singular: the window.

If it were plural, you would expect a different form, for example:

• glugga — windows (accusative plural, indefinite)
• gluggana — the windows (accusative plural, definite)

So út um gluggann specifically means out through the window, not through the windows.

Usually the most natural meaning is the habitual one:

• We look out the window when it rains.

In the right context, Icelandic present tense can sometimes overlap with English are looking, but without extra context this sentence is best understood as a general statement or habit.

If you wanted to make the action feel more clearly ongoing in context, Icelandic would usually rely on other wording or context rather than a special progressive verb form like English does.

In normal Icelandic, við is usually included.

Unlike some languages that regularly drop subject pronouns, Icelandic normally keeps them:

• Við horfum — We look

Because the pronoun is present, the sentence sounds complete and natural. The verb ending already helps show the person and number, but Icelandic still generally uses the subject pronoun in ordinary statements.