Únik vody byl tak velký, že je nábytek zničený.

Because únik vody uses the genitive case. After nouns like únik meaning leak, escape, or loss, Czech often puts the thing involved in the genitive.

So:

• únik vody = leak of water / water leak
• literally, something like escape of water

This is very common in Czech. Similar patterns are:

• sklenice vody = a glass of water
• láhev vína = a bottle of wine

So vody is not random; it is the normal form after únik.

They agree with únik, which is a masculine singular noun.

• únik = masculine singular
• therefore byl = masculine singular past tense of být
• and velký = masculine singular adjective

So the grammar is:

• Únik vody byl tak velký...
• The water leak was so big...

If the noun were feminine, the forms would change. For example:

• havárie byla tak velká...
  because havárie is feminine

Tak ... že means so ... that.

In this sentence:

• tak velký = so big
• že = that

So:

• Únik vody byl tak velký, že...
• The leak was so big that...

This is a very common Czech pattern:

• Byl tak unavený, že usnul. = He was so tired that he fell asleep.
• Mluvila tak rychle, že jsem jí nerozuměl. = She spoke so fast that I couldn’t understand her.

This usually means:

• the leak happened in the past
• but the result is still true now

So the sentence suggests something like:

• the leak was very large
• and as a result, the furniture is now destroyed

That tense combination is possible in Czech, just as it is in English.

If you wanted the whole situation to stay in the past, you could say:

• Únik vody byl tak velký, že byl nábytek zničený.

If you want to stress the present result, je zničený makes sense.

Because nábytek in Czech is a singular collective/mass noun, much like furniture in English.

So Czech says:

• nábytek je zničený = the furniture is destroyed

not:

• nábytek jsou zničené

Even if the furniture consists of many pieces, the noun itself is grammatically singular.

Because Czech word order is flexible. It is not as fixed as English word order.

Both of these are possible:

• že je nábytek zničený
• že nábytek je zničený

The difference is mostly about emphasis and information structure, not basic meaning.

The version in your sentence sounds natural and neutral. Czech often moves words around depending on what is already known and what is new or important.

So as a learner, the main thing to remember is:

• both can be correct
• Czech word order often reflects emphasis more than strict grammar rules

Zničený comes from the verb zničit, meaning to destroy or to ruin.

Here it functions like an adjective/past participle:

• zničený nábytek = destroyed furniture
• nábytek je zničený = the furniture is destroyed

With je, it describes the state/result.

So:

• je zničený = is destroyed / is ruined

In everyday Czech, this long form zničený is very normal.

Yes, but the difference is mostly about style and sometimes a slight nuance.

• je zničený sounds very natural in everyday Czech
• je zničen is more formal or written

In many contexts, they mean almost the same thing:

• Nábytek je zničený.
• Nábytek je zničen.

A learner will hear the -ý form very often in normal speech.

Because in Czech, a subordinate clause introduced by že normally takes a comma.

So:

• Únik vody byl tak velký, že je nábytek zničený.

That comma is required in standard Czech punctuation.

This is similar to how Czech also uses commas before other subordinating conjunctions such as:

• protože
• když
• aby

It is understandable, but depending on context, a Czech speaker might also say it in a more direct way.

For example:

• Únik vody byl tak velký, že zničil nábytek.
  = The leak was so big that it destroyed the furniture.

This version focuses more on the action and may sound more straightforward.

The original sentence is still useful, especially if you want to emphasize the current result:

• the furniture is destroyed now

So the sentence is grammatical and meaningful; there are just other natural ways to express the same idea.