Hloubka je velká.

Because velká has to agree with hloubka.

In Czech, adjectives agree with the noun they describe in:

• gender
• number
• case

Hloubka is feminine singular nominative, so the adjective also appears in the feminine singular nominative form: velká.

Compare:

• problém je velký = the problem is big/great (masculine)
• město je velké = the city/town is big (neuter)
• hloubka je velká = the depth is great (feminine)

A very common clue is the ending -a in the dictionary form (nominative singular). Most Czech nouns ending in -a are feminine.

So:

• hloubka = feminine

That is why it pairs with feminine forms like:

• velká
• malá
• hluboká

As always, there are exceptions in Czech, but for a noun like hloubka, feminine is exactly what you would expect.

Because the noun here is hloubka = depth, and Czech normally describes depth as being large/great rather than deep.

So:

• hloubka je velká = the depth is great / the depth is large

By contrast, hluboký / hluboká / hluboké means deep, and it is usually used with things like:

• řeka je hluboká = the river is deep
• díra je hluboká = the hole is deep

In other words:

• something can be deep
• its depth can be great/large

That works much like English.

Hloubka is in the nominative case because it is the subject of the sentence.

The sentence structure is:

• hloubka = subject
• je = is
• velká = predicate adjective

After být (to be), the adjective is also normally in the nominative, so velká is nominative too.

So both main words are nominative here:

• hloubka — nominative singular
• velká — nominative singular feminine

Je is the 3rd person singular present tense of být = to be.

So:

• je = is

Examples:

• To je dobré. = That is good.
• Dům je velký. = The house is big.
• Hloubka je velká. = The depth is great.

Unlike in some other Slavic languages, standard Czech normally does use the present-tense form of to be, so you do not usually leave je out.

Czech has no articles, so there is no direct equivalent of a or the in ordinary sentences.

That means Hloubka je velká can be understood from context as:

• The depth is great
• or more generally Depth is great, depending on the situation

Usually the context tells you whether the meaning is definite or indefinite.

If Czech wants to be more specific, it may use other words, for example:

• ta hloubka = that depth / the depth (in context)

But there is no separate word that works exactly like English the.

The most neutral word order is:

• Hloubka je velká.

Czech word order is fairly flexible, but changing it usually changes the emphasis or makes the sentence sound more marked.

For example:

• Velká je hloubka. — possible, but unusual in everyday neutral speech; it sounds emphatic, literary, or contrastive

So for a basic statement, Hloubka je velká is the safest and most natural version.

A rough English-friendly approximation is:

HLOUB-kah yeh VEL-kah

A few useful details:

• Czech stress is usually on the first syllable of each word
• je sounds like yeh
• ou in hloubka is a real diphthong, roughly like saying oh
  • oo quickly together
• á is a long vowel, so velká has a longer final a sound than short a

If you want a simpler pronunciation guide:

• hloubka ≈ HLOHB-kah
• je ≈ yeh
• velká ≈ VEL-kah

Yes. It is still an adjective.

Here it is used as a predicate adjective, not directly before a noun.

Compare:

• velká hloubka = a great/large depth
  Here the adjective stands directly before the noun.

• hloubka je velká = the depth is great/large
  Here the adjective comes after je, but it is still an adjective and still agrees with hloubka.

English works similarly:

• a big house
• the house is big

You make it negative by changing je to není:

• Hloubka není velká. = The depth is not great / not large.

This is very common in Czech:

• je = is
• není = is not

Examples:

• Dům je velký. = The house is big.
• Dům není velký. = The house is not big.

So the same pattern works here:

• Hloubka je velká.
• Hloubka není velká.