Ovaj vrč je glinen.

Because ovaj has to agree with vrč.

Here, vrč is:

• masculine
• singular
• nominative

So the matching form of this is ovaj.

Compare:

• ovaj vrč = this jug/pitcher
• ova zdjela = this bowl
• ovo selo = this village

Croatian demonstratives change form depending on gender, number, and case.

It is nominative singular.

That is because vrč is the subject of the sentence: it is the thing being described as glinen.

A simple way to see the structure is:

• Ovaj vrč = the subject
• je = is
• glinen = made of clay / clay

So vrč stays in the nominative.

Usually, yes. In Croatian, many nouns ending in a consonant in the nominative singular are masculine, and vrč follows that pattern.

So:

• vrč is masculine
• that is why you get ovaj vrč
• and that is also why the adjective appears as glinen in the masculine singular form

This is a very common pattern, though of course there are some exceptions elsewhere in the language.

Je is the 3rd person singular present tense of biti, meaning to be.

So here it means is.

• ja sam = I am
• ti si = you are
• on / ona / ono je = he / she / it is

In Ovaj vrč je glinen, je links the subject ovaj vrč with the description glinen.

Because je is a clitic in Croatian. Clitics usually go in second position in the clause.

In this sentence, the first whole unit is Ovaj vrč, so je comes right after it:

• Ovaj vrč je glinen.

That is the normal neutral order.

A form like Ovaj vrč glinen je sounds marked, poetic, or unnatural in ordinary speech.

This is a very common learner question.

Glinen is the short masculine singular form used in predicate position, especially after je:

• Vrč je glinen.

Before a noun, you will often see the long form:

• glineni vrč

So both forms are related, but they are used differently.

A useful rule of thumb:

• after the verb to be: glinen
• before the noun: glineni

Not every Croatian adjective shows this contrast in the same way, but this one does.

Glinen is an adjective meaning made of clay, clay, or earthen.

It comes from the noun glina = clay.

So:

• glina = clay
• glinen = clay / made of clay

It does not function here as a noun. It is describing what the jug is made of.

Because Croatian does not have articles like English.

So Croatian does not normally use separate words corresponding to a or the.

Instead, definiteness is understood from:

• context
• word order
• demonstratives such as ovaj = this

In this sentence, ovaj already makes the noun specific.

Vrč is pronounced as one syllable.

Important points:

• v is like English v
• r can act like a vowel in Croatian, so there is no written vowel between v and r
• č sounds like the ch in church, but a bit firmer

A rough English-friendly approximation is vrch.

If you want the technical pronunciation, it is approximately /ʋr̩t͡ʃ/.

Yes. That is also natural Croatian.

• Ovaj vrč je glinen.
• Ovaj vrč je od gline.

Both mean that the jug is made of clay.

The difference is mainly style:

• glinen = adjectival, compact
• od gline = more literal, explicitly of clay

Both are good.

The words have to agree with the noun.

Examples:

• Ova zdjela je glinena.
  feminine singular

• Ovo posuđe je glineno.
  neuter singular

• Ovi vrčevi su glineni.
  masculine plural

Notice two things:

1. ovaj / ova / ovo / ovi changes
2. glinen / glinena / glineno / glineni changes
3. in the plural, je becomes su

So agreement is doing a lot of work in this sentence.