Ущерб оказался серьёзным.

Because after verbs like быть, стать, and оказаться, Russian often uses the instrumental for the predicate noun or adjective.

So here:

• ущерб = the subject, in the nominative
• серьёзным = the description of that subject, in the instrumental

This is very common in Russian:

• Он был врачом. = He was a doctor.
• Проблема стала серьёзной. = The problem became serious.
• Ущерб оказался серьёзным. = The damage turned out to be serious.

If you were using the adjective directly before the noun, then it would be nominative:

• серьёзный ущерб = serious damage

So серьёзным is not modifying the noun inside a noun phrase; it is part of the sentence predicate.

Оказался means turned out to be or proved to be.

That is different from был, which just means was.

Compare:

• Ущерб был серьёзным. = The damage was serious.
• Ущерб оказался серьёзным. = The damage turned out to be serious.

The second version suggests some kind of discovery, result, or realization. Maybe at first people did not know how bad the damage was, and later it became clear.

So оказался adds the idea of unexpected finding or revealed reality.

Because it agrees with ущерб, which is a masculine singular noun.

In the past tense, Russian verbs agree with the subject in gender and number:

• masculine: оказался
• feminine: оказалась
• neuter: оказалось
• plural: оказались

Since ущерб is masculine singular, the verb is оказался.

Ущерб is in the nominative singular because it is the subject of the sentence.

It is the thing that turned out to be serious.

Many masculine nouns in Russian have a zero ending in the nominative singular, so the form looks plain:

• ущерб = nominative singular
• ущерба = genitive singular
• ущербу = dative singular
• ущербом = instrumental singular

Yes, оказаться is perfective.

That means it focuses on the result: the moment something becomes clear or is revealed.

Its past tense is оказался, as in your sentence.

Because it is perfective, it does not have a true present tense. Forms like окажется mean future, not present:

• Он окажется прав. = He will turn out to be right.

The imperfective partner is оказываться:

• Он оказывается прав. = He turns out to be right / is turning out to be right.
• Он оказывался прав. = He used to turn out to be right / turned out to be right repeatedly or in context

So in your sentence, the perfective verb fits well because it presents the seriousness as a discovered fact.

Not really in a literal self sense.

In оказаться, the -ся is part of the verb as a whole. You should learn оказаться as its own dictionary form, not as just оказать + ся in a transparent, literal way.

For learners, the most useful thing is to remember the pair:

• оказываться = imperfective
• оказаться = perfective

And the common meaning:

• оказаться + instrumental / noun = to turn out to be

So here -ся is not best understood as a separate reflexive meaning like wash oneself.

Because after оказаться, the most neutral and natural pattern is usually:

subject + оказался/оказалась/оказалось + full adjective in the instrumental

So:

• Ущерб оказался серьёзным.

This is standard Russian.

Short adjectives do exist, and some are common after verbs like быть or оказаться, for example:

• Он оказался прав. = He turned out to be right.
• Она оказалась готова. = She turned out to be ready.

But with серьёзный, the full instrumental form серьёзным is the natural neutral choice here.

Yes. Russian word order is flexible, and changing it changes the emphasis more than the basic meaning.

The neutral order is:

• Ущерб оказался серьёзным.

If you move серьёзным forward, you emphasize it:

• Серьёзным оказался ущерб.

That sounds more like: It was serious, the damage turned out to be.

So the original sentence is the most neutral, textbook-like version.

Yes, but the meaning and style are slightly different.

• Ущерб оказался серьёзным.
  The damage turned out to be serious.
  There is an idea of discovery.

• Ущерб был серьёзным.
  The damage was serious.
  This is a plain statement in the past.

• Ущерб серьёзный.
  The damage is serious.
  This is present-time or general statement style, with the present-tense to be omitted, as Russian normally does.

So all three are possible, but they are not interchangeable in nuance.

It agrees with ущерб in gender and number, but its case is determined by its role in the sentence.

Here:

• ущерб = masculine singular nominative
• серьёзным = masculine singular instrumental

The adjective still matches the noun as masculine singular, but because it is a predicate complement after оказался, it appears in the instrumental.

So agreement does not always mean same case in this kind of structure.

The stress is:

• уще́рб? Wait, no: actually уще́рб is not correct.
• Correct: уще́рб? Let's give it clearly as standard stress:
• ущерб — stress on the second syllable: уще́рб?
  More practically: say it as u-SHCHERB.
• оказа́лся — stress on -зал-: a-ka-ZAL-sya
• серьёзным — stress on ё: ser-YOZ-nym

A useful rule: ё is always stressed.

So the whole sentence sounds approximately like:

u-SHCHERB a-ka-ZAL-sya ser-YOZ-nym

These words overlap, but they are not identical.

• ущерб = damage, harm, loss; often fairly broad, and often used for material, economic, or overall damage
• вред = harm; often more about harmful effect, including health or environment
• убытки = losses; usually specifically financial losses

Examples:

• нанести ущерб компании = to cause damage to a company
• вред для здоровья = harm to health
• финансовые убытки = financial losses

So ущерб оказался серьёзным is a natural way to say that the damage/loss turned out to be serious.