Η απειλή είναι σοβαρή.

Η is the definite article meaning the.

In this sentence:

• Η = the
• απειλή = threat

So Η απειλή means the threat.

Greek articles change depending on gender, number, and case, so η is specifically the feminine singular nominative form.

In Greek, every noun has a grammatical gender: masculine, feminine, or neuter.
Απειλή happens to be a feminine noun, so it takes:

• the feminine article η
• the feminine adjective form σοβαρή

This is grammatical gender, not biological gender. A word like threat is not “female” in meaning; it simply belongs to the feminine grammatical class in Greek.

Because Greek adjectives must agree with the noun they describe.

Since απειλή is:

• feminine
• singular
• nominative

the adjective must also be:

• feminine
• singular
• nominative

So you get:

• masculine: σοβαρός
• feminine: σοβαρή
• neuter: σοβαρό

Here, σοβαρή matches απειλή.

Είναι means is (or are, depending on context). It is the present-tense form of the verb είμαι = to be.

In this sentence:

• Η απειλή = the threat
• είναι = is
• σοβαρή = serious

So the structure is very close to English:
The threat is serious.

Unlike in some languages, standard Greek normally does use the verb to be in sentences like this.

It can mean both.

In the present tense, είναι is used for both:

• he/she/it is
• they are

So by itself, είναι does not tell you whether the subject is singular or plural. You understand that from the subject:

• Η απειλή είναι σοβαρή. = The threat is serious.
• Οι απειλές είναι σοβαρές. = The threats are serious.

A natural pronunciation is approximately:

ee ah-pee-LEE EE-neh so-vah-REE

More closely:

• Η = ee
• απειλή = ah-pee-LEE
• είναι = EE-neh
• σοβαρή = so-vah-REE

The stress falls on:

• απειλή
• είναι
• σοβαρή

So the sentence sounds roughly like:

ee ah-pee-LEE EE-neh so-vah-REE

The accent marks show which syllable is stressed.

In Modern Greek, stress matters in pronunciation, so the accent helps you say the word correctly:

• απειλή
• είναι
• σοβαρή

The article η has no accent here because it is a very short function word and does not need one in this form.

In Greek, adjectives can often appear after or before the noun, but the position can affect style or emphasis.

Here:

• Η απειλή είναι σοβαρή.

the adjective σοβαρή comes after είναι because it is a predicate adjective, not directly attached before the noun.

Compare:

• Η απειλή είναι σοβαρή. = The threat is serious.
• Η σοβαρή απειλή... = The serious threat...

So in your sentence, σοβαρή is not just modifying the noun inside the noun phrase; it is part of the statement about the noun.

In standard Modern Greek, normally no. You would usually keep είναι in a sentence like this.

So:

• Η απειλή είναι σοβαρή. = normal and correct

Leaving it out would sound incomplete in standard usage.

Greek often drops subject pronouns like εγώ or αυτός, but not usually the verb to be in this kind of sentence.

Yes, Greek word order is more flexible than English, so that is possible.

• Η απειλή είναι σοβαρή. = neutral, standard order
• Σοβαρή είναι η απειλή. = more emphasis on serious
• Η σοβαρή απειλή = means the serious threat, which is a different structure

So changing the order is possible, but it changes the focus or emphasis.

It would become:

Οι απειλές είναι σοβαρές. = The threats are serious.

Notice the changes:

• Η → Οι = the
• απειλή → απειλές = threat → threats
• σοβαρή → σοβαρές = serious (feminine singular → feminine plural)

The verb stays:

• είναι = is / are

So this is a good example of how Greek shows number mostly through the article, noun, and adjective, not by changing είναι.

Grammatically, μια απειλή means a threat, yes. But whether the full sentence sounds natural depends on context.

• Η απειλή είναι σοβαρή. = The threat is serious.
• Μια απειλή είναι σοβαρή. = A threat is serious.

The second sentence can sound more general or context-dependent in English too. Very often, if you are talking about one specific threat, Greek prefers η απειλή.

So the article matters a lot:

• η = the
• μια = a / one

It is neutral standard Greek. It sounds natural in everyday language, news language, and formal contexts.

So Η απειλή είναι σοβαρή. is a very normal sentence and not unusually formal or literary.

It is in the nominative case, because it is the subject of the sentence.

You can tell from:

• the article η
• the basic dictionary form of the noun
• the fact that it is the thing being described as serious

So:

• Η απειλή = nominative singular feminine
• σοβαρή also appears in the matching nominative feminine singular form

This is the basic subject pattern learners will see very often in Greek.