Göngubrúin er ekki eins löng og ég hélt, en hún er öruggari en gangbrautin.

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

• göngubrú = footbridge
• göngubrúin = the footbridge
• gangbraut = pedestrian crossing / crosswalk
• gangbrautin = the pedestrian crossing / the crosswalk

In this sentence, both are feminine singular forms with the suffixed definite article.

The adjective has to match the noun in gender and number.

The noun göngubrúin is feminine singular, so the adjective langur changes to the feminine form löng.

Compare:

• masculine: langur
• feminine: löng
• neuter: langt

So Göngubrúin er löng means The footbridge is long.

That is normal Icelandic word order. In a main clause, the finite verb usually comes before the negation.

So:

• er ekki = is not

This is a very common pattern:

• Hann er ekki hér = He is not here
• Hún kemur ekki = She is not coming

The pattern is:

ekki eins + adjective + og

So here:

• ekki eins löng og = not as long as

More generally:

• eins stór og = as big as
• ekki eins stór og = not as big as

In this structure, og means as, not the ordinary and.

Yes, og usually means and, but after eins it is part of the comparison pattern eins ... og = as ... as.

So in this sentence:

• eins löng og ég hélt = as long as I thought

With the negation:

• ekki eins löng og ég hélt = not as long as I thought

So this is not the coordinating og meaning and.

It is a shortened clause. Icelandic often leaves out words that are easy to understand from context.

A fuller version would be something like:

• ekki eins löng og ég hélt að hún væri

That means not as long as I thought it was.

So ég hélt on its own here works like English I thought in a shortened comparison.

This is the comparative pattern.

• öruggur = safe / secure
• öruggari = safer
• en = than

So:

• öruggari en gangbrautin = safer than the crosswalk

This is the normal Icelandic way to make a more ... than ... comparison.

They are doing two different comparison jobs.

• eins ... og = as ... as
• comparative + en = ...er than

So:

• ekki eins löng og ég hélt = not as long as I thought
• öruggari en gangbrautin = safer than the crosswalk

A useful shortcut is:

• og after eins
• en after a comparative

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

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

So hún here means it, referring back to göngubrúin.

This is very common in Icelandic: things are often referred to with hann, hún, or það depending on the noun’s gender.

They are related, but they are not the same thing.

• göngubrú = footbridge / pedestrian bridge
• gangbraut = pedestrian crossing / crosswalk

So a göngubrú goes over something, while a gangbraut is a place where pedestrians cross at street level.

Because the sentence has two clauses joined by en = but:

• Göngubrúin er ekki eins löng og ég hélt
• en hún er öruggari en gangbrautin

Repeating er is completely normal. English does the same thing:

• The footbridge isn’t as long as I thought, but it is safer than the crosswalk.

Yes. Icelandic makes a lot of vocabulary by combining smaller words into compounds.

• göngubrú = roughly walking + bridge
• gangbraut = roughly walking/pedestrian + way/path

This is extremely common in Icelandic, so learning to recognize compounds is very helpful. Also, the first part of a compound does not always look exactly like the dictionary form you might expect, so it is often best to learn the whole compound as a unit.