El descobert és greu.

Because el descobert is being used as a noun phrase, not just as a bare adjective or participle.

In Catalan, it is very common to use el + adjective/past participle to mean the ... thing / the ... situation / the ... concept. So here el helps turn descobert into something noun-like.

So structurally:

• el descobert = the noun phrase
• és greu = is serious

In many contexts, el descobert is a fixed expression, so the article is part of how the phrase is normally said.

Here, it functions as a noun.

On its own, descobert is originally the past participle of descobrir. But in this sentence it is being used substantively, meaning it behaves like a noun.

That is why the sentence works like:

• El descobert = subject
• és = verb
• greu = adjective describing the subject

So even if the word historically comes from a participle, in this sentence you should treat el descobert as a noun phrase.

Because Catalan usually uses ser for something seen as an essential characteristic, definition, or general judgment, and estar more for a state or condition.

Here, greu is being presented as a judgment about the subject, so és greu is the natural choice.

Compare the general idea:

• és greu = it is serious
• està greu = it is in a serious condition

With people or medical situations, està greu is common. But in a sentence like this, where you are classifying or evaluating something, és greu is the expected form.

Because in Catalan, adjectives often come after the noun, especially when they describe or classify it in a straightforward way.

Here the pattern is:

• subject: El descobert
• verb: és
• adjective/complement: greu

So this is not really “noun + adjective” inside one noun phrase. It is a full sentence:

• El descobert = the subject
• és greu = says what it is like

That word order is completely normal in Catalan.

Because greu is one of those adjectives that has the same singular form for masculine and feminine.

So:

• masculine singular: greu
• feminine singular: greu
• plural: greus

Since el descobert is singular, greu stays singular too.

The accent mark distinguishes és from es.

• és = is (third person singular of ser)
• es = a reflexive/pronominal pronoun, like himself/herself/itself, depending on context

So the accent is important because it marks a completely different word.

Yes. Descobert is the past participle of descobrir.

Very roughly:

• descobrir = to uncover / discover
• descobert = discovered / uncovered

But in this sentence, you should not read it simply as a participle. It has become a noun-like expression, especially with the article el.

This kind of development is very common in Catalan and other Romance languages.

Not in normal Catalan.

If descobert is the subject as a noun phrase, you normally need the article:

• El descobert és greu.

Without el, the sentence sounds incomplete or unnatural in this meaning.

Catalan often uses the definite article where English might not, and in this case it is part of the normal structure.

The normal, neutral order is:

• El descobert és greu.

You can change word order in Catalan for emphasis, but it would sound marked or literary, not like the default way to say it.

So for a learner, the safe pattern is:

• subject + verb + complement

That is exactly what this sentence does.

A simple learner-friendly approximation is:

el des-ku-BERT es GREU

A few notes:

• descobert is stressed on the last syllable: -bert
• és is a short, stressed word
• greu is one syllable in standard pronunciation, with a diphthong at the end

Pronunciation varies a bit by dialect, but the stress pattern above is the important part for learners.