Насколько серьёзна протечка?

In Russian, the verb to be is usually omitted in the present tense.

So English How serious is the leak? becomes literally something like:

• Насколько серьёзна протечка?
• How serious the leak?

That is completely normal Russian.
If you put the sentence into the past or future, a form of to be appears:

• Насколько серьёзной была протечка? — How serious was the leak?
• Насколько серьёзной будет протечка? — How serious will the leak be?

Серьёзна is the short form of the adjective серьёзный.

Russian often uses the short form of an adjective when the adjective is the main predicate of the sentence — that is, when it means is serious rather than directly describing the noun inside a noun phrase.

Here:

• протечка = the leak
• серьёзна = is serious

So серьёзна works like a predicate adjective.

The full form серьёзная is also possible in some statements, for example:

• Протечка серьёзная. — The leak is serious.

But in a question like Насколько серьёзна протечка?, the short form sounds especially standard and natural.

Because it agrees with протечка, which is feminine singular.

The noun протечка is feminine, so the short adjective must also be feminine:

• masculine: серьёзен
• feminine: серьёзна
• neuter: серьёзно
• plural: серьёзны

So:

• протечка → feminine
• therefore серьёзна → feminine

Here насколько means how, to what extent, or how much in the sense of degree.

So the sentence is asking about the degree of seriousness:

• Насколько серьёзна протечка?
• How serious is the leak?
• literally: To what extent is the leak serious?

This word is very common when asking about degree:

• Насколько это важно? — How important is this?
• Насколько это сложно? — How difficult is this?

Usually насколько is the more natural choice here.

• Насколько серьёзна протечка? — standard, natural
• Как серьёзна протечка? — possible, but less common and can sound more literary or less idiomatic

For degree questions like how serious, how important, how difficult, Russian very often prefers насколько.

Протечка is in the nominative singular.

It is the subject of the sentence — the thing being described as serious.

So the structure is basically:

• Насколько — to what extent
• серьёзна — is serious
• протечка — the leak

The noun stays in the nominative because it is the subject, not a direct object or prepositional phrase.

Russian word order is more flexible than English, but this version is the most neutral:

• Насколько серьёзна протечка?

You may also hear or see other orders for emphasis, for example:

• Протечка насколько серьёзна? — marked, unusual in normal conversation
• Насколько протечка серьёзна? — possible, but less natural than the standard order

For a learner, the safest version is definitely:

• Насколько серьёзна протечка?

The stresses are:

• наскОлько
• серьЁзна
• протЕчка

So:

• НаскОлько серьЁзна протЕчка?

A useful detail: the letter ё is always stressed, so in серьёзна the stress must fall on ё.

A rough English-style approximation would be:

• na-SKOL-ka sir-YOZ-na pra-TYECH-ka

But it is better to listen to native audio if possible, especially for Russian vowel reduction.

It sounds neutral to slightly formal.

It would fit well in contexts like:

• a plumber talking to a customer
• a building maintenance report
• a technical conversation
• a concerned but normal everyday question

If you wanted something more casual in conversation, you might hear:

• Насколько всё плохо? — How bad is it?
• Сильно течёт? — Is it leaking badly?

But Насколько серьёзна протечка? is perfectly normal and natural.

Протечка usually means a physical leak, especially involving water or a building/pipe/roof problem.

Typical examples:

• a leaking pipe
• a roof leak
• water seeping through a ceiling

This is different from some other Russian words that English translates as leak:

• утечка газа — gas leak
• утечка информации — information leak
• утечка данных — data leak

So протечка is the right word for the kind of leak you would normally find in plumbing or a building.

Yes, a useful literal breakdown is:

• насколько — to what extent / how
• серьёзна — serious-is / is serious
• протечка — leak

So the whole sentence is structurally close to:

• To what extent is the leak serious?

That literal view helps explain both the short adjective серьёзна and the lack of a separate word for is.