Gardínan er of stutt, svo hún er ekki falleg í glugganum.

Because Icelandic usually puts the definite article onto the end of the noun.

• gardína = curtain
• gardínan = the curtain

Here, -an is the definite ending for this form of a feminine noun.

Also, the word is capitalized only because it starts the sentence. In the middle of a sentence, it would normally be gardínan.

Gardína is a feminine noun. That matters because Icelandic adjectives and pronouns often have to agree with the noun in gender, number, and case.

So in this sentence:

• gardínan is feminine singular
• hún is the feminine pronoun she/it
• falleg is the feminine form of beautiful
• stutt is also the correct agreeing form here

In Icelandic, even objects are referred to by grammatical gender, not by natural gender.

Because the adjective has to agree with gardínan.

The adjective is stuttur = short, but that is the masculine nominative singular form.
Here the noun is feminine singular nominative, so the correct form is stutt.

A few forms of stuttur are:

• masculine: stuttur
• feminine: stutt
• neuter: stutt

So even though stutt may look like a neuter form if you are used to other adjectives, it is also the correct feminine form here.

Of means too.

So:

• of stutt = too short

It is used before adjectives and adverbs to show excess:

• of dýr = too expensive
• of stórt = too big

This is a very common word in Icelandic.

Here svo means so in the sense of and therefore / as a result.

So the sentence structure is:

• Gardínan er of stutt = first statement
• svo hún er ekki falleg í glugganum = result/consequence

Be aware that svo can have other meanings in other contexts too, such as so, then, or very, depending on the sentence.

Because gardína is a feminine noun, and Icelandic pronouns follow grammatical gender.

So:

• masculine noun → often hann
• feminine noun → often hún
• neuter noun → often það

In English, we usually say it for objects. In Icelandic, the pronoun depends on the noun’s grammatical gender, so hún is completely normal here.

Again, it is because of agreement.

The adjective fallegur = beautiful changes form depending on the noun:

• masculine: fallegur
• feminine: falleg
• neuter: fallegt

Since gardínan is feminine singular, the correct form is falleg.

So:

• Gardínan er falleg. = The curtain is beautiful.

Glugganum is the dative singular definite form of gluggi = window.

Breakdown:

• gluggi = window
• glugganum = the window in dative singular

The preposition í can take either:

• accusative for movement into something
• dative for location in something

Here it means location: in the window, not motion into the window, so Icelandic uses the dative.

Compare:

• í gluggann = into the window / to the window opening (movement)
• í glugganum = in the window (location)

Because there are two clauses, and each clause needs its own finite verb.

• Gardínan er of stutt
• svo hún er ekki falleg í glugganum

English works the same way here:

• The curtain is too short, so it is not pretty in the window.

You cannot normally leave out the second er in Icelandic.

Yes. This is normal main-clause word order:

• svo hún er ekki falleg í glugganum

That is:

• conjunction
• subject (hún)
• verb (er)
• negation/adjective/etc.

Icelandic often follows verb-second patterns, but after a conjunction like svo, this kind of order is very common and natural.

Because ekki negates the predicate.

So:

• hún er falleg = it is beautiful
• hún er ekki falleg = it is not beautiful

In Icelandic, ekki usually comes after the finite verb and before the part being negated, which is exactly what happens here.

Yes, and that is exactly what the sentence does. Icelandic often avoids repeating the noun once the reference is clear.

So instead of saying:

• Gardínan er of stutt, svo gardínan er ekki falleg í glugganum

it is much more natural to say:

• Gardínan er of stutt, svo hún er ekki falleg í glugganum.

Using the pronoun sounds smoother and more natural.