Oštećenje je dugo jedan metar.

Because oštećenje is a neuter singular noun, and the adjective has to agree with it.

The adjective dug changes by gender:

• masculine: dug
• feminine: duga
• neuter: dugo

So:

• most je dug = the bridge is long
• cesta je duga = the road is long
• oštećenje je dugo = the damage/damaged area is long

Here it is an adjective, not an adverb.

That can be confusing because dugo is also the adverb form meaning for a long time / long in sentences like:

• Čekao sam dugo. = I waited a long time.

But in Oštećenje je dugo jedan metar, dugo describes oštećenje, so it is an adjective agreeing with the noun.

Je is the 3rd person singular of biti = to be, and in Croatian it behaves like a clitic. Clitics usually come in the second position in the sentence or clause.

So in:

• Oštećenje je dugo jedan metar.

the first element is Oštećenje, and je comes right after it.

This second-position behavior is very common in Croatian.

With measurements after adjectives like dug (long), širok (wide), visok (high/tall), Croatian commonly uses a measure expression without a preposition.

So you get patterns like:

• dug jedan metar
• širok dva metra
• visok tri metra

In this sentence, jedan metar is the measurement phrase. Also, with metar, the singular nominative and accusative look the same, so metar does not change form here.

Compare:

• jedan metar
• dva metra
• pet metara

Not always.

Croatian often omits jedan in measurement expressions when English would still say one:

• Oštećenje je metar dugo.
• Oštećenje je jedan metar dugo.

Both are possible.
Using jedan can sound a bit more explicit, exact, or contrastive.

Yes, you can also say Oštećenje je jedan metar dugo, and many learners will hear that order very often.

Croatian word order is fairly flexible, especially with measurement phrases. So both of these work:

• Oštećenje je dugo jedan metar.
• Oštećenje je jedan metar dugo.

The second version may sound a bit more neutral or common to many speakers, but the original sentence is understandable and grammatical.

Yes, you may hear dugačak / dugačka / dugačko as well.

So a similar sentence could be:

• Oštećenje je dugačko jedan metar.

But with exact measurements, dug is very common and standard:

• jedan metar dug
• dva metra dug
• pet metara dug

So dugo in your sentence is perfectly normal.

In a sentence like this, oštećenje usually refers to a specific damaged area, defect, or damaged section, not just the abstract idea of damage in general.

That is why it can be described with a physical measurement like one meter long.

So the sentence is talking about the extent of the damaged part.

Because Croatian does not have articles like a/an and the.

So oštećenje can mean:

• damage
• the damage
• a damaged area

The exact interpretation depends on context.

That is normal in Croatian, and learners have to get used to reading definiteness from the situation rather than from an article.