Ovaj park je ljepši u proljeće.

Ovaj means this.

It matches park in:

• gender: masculine
• number: singular
• case: nominative

So ovaj park = this park.

Croatian has a few common demonstratives:

• ovaj = this
• taj = that
• onaj = that over there / that one

Because the noun is park and it is masculine singular, the correct form is ovaj.

Yes, park is masculine.

A very common pattern in Croatian is:

• nouns ending in a consonant are usually masculine
• nouns ending in -a are often feminine
• nouns ending in -o or -e are often neuter

So park ends in a consonant, which strongly suggests masculine.

In this sentence, park is the subject, so it appears in the nominative singular: park.

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

So:

• ovaj park je ljepši = this park is prettier

Why is it there? Because je is a clitic in Croatian. Clitics usually go in the second position of the sentence or clause.

Here, the first unit is Ovaj park, so je comes right after it:

• Ovaj park je ljepši u proljeće.

You will often see this with forms like je, su, sam, si, smo, ste.

Ljepši is the comparative of lijep.

• lijep = beautiful / pretty / nice
• ljepši = more beautiful / prettier / nicer

So je ljepši means is prettier or is nicer.

This comparative form also agrees with park, so it is:

• masculine
• singular
• nominative

Compare:

• Ovaj park je ljepši. = masculine singular
• Ova ulica je ljepša. = feminine singular
• Ovo jezero je ljepše. = neuter singular

The change from lijep to ljepši is not fully predictable from English, so it is best learned as a vocabulary pattern:

• lijep → ljepši

Because Croatian usually makes the comparative inside the adjective itself.

English can say:

• pretty → prettier
• beautiful → more beautiful

Croatian usually prefers a single comparative form:

• lijep → ljepši

So instead of something like više lijep, Croatian normally uses ljepši.

That is why:

• Ovaj park je ljepši
  not normally
• Ovaj park je više lijep

In this sentence, u proljeće means in spring as a time expression.

With seasons and some other time expressions, Croatian commonly uses u + accusative:

• u proljeće = in spring
• u jesen = in autumn
• u nedjelju = on Sunday

Here, proljeće is a neuter noun, and the form proljeće is the accusative singular.

Learners often expect u + locative, because u also means in for location. But here it is not about physical location; it is a time frame, and Croatian uses u proljeće.

So:

• u proljeće = correct here

Good question. Croatian, like English, can use a comparative without stating the second half explicitly.

So Ovaj park je ljepši u proljeće naturally implies something like:

• prettier than in other seasons
• prettier than usual
• prettier at that time of year

The comparison is understood from context.

If you want to say the comparison explicitly, you can add it:

• Ovaj park je ljepši u proljeće nego zimi.
  = This park is prettier in spring than in winter.

Yes, Croatian word order is more flexible than English word order.

The original sentence is the most neutral:

• Ovaj park je ljepši u proljeće.

But you could also say:

• U proljeće je ovaj park ljepši.

That version puts more focus on u proljeće.

One important thing stays true: the clitic je usually comes in second position in the clause. So when U proljeće comes first, je follows it:

• U proljeće je ovaj park ljepši.

A rough English-friendly guide:

• lj is a single Croatian sound. It is somewhat similar to the lli in million for many English speakers, though not exactly the same.
• je is pronounced roughly like ye
• š sounds like sh
• ć is a softer sound than č, roughly somewhere near ty/ch for English ears

So approximately:

• ljepši ≈ LYEP-shee
• proljeće ≈ PRO-lye-cheh or PRO-lye-tyeh as a rough guide

And the whole sentence can be approximated as:

• OH-vai park ye LYEP-shee oo PRO-lye-cheh

That is only an approximation, but it is a useful starting point.

Because Croatian does not have articles like English a and the.

Instead, Croatian uses context, word order, and words like demonstratives when needed.

So:

• park can mean a park or the park, depending on context
• ovaj park clearly means this park

That is why the sentence does not need a separate word for the:

• Ovaj park je ljepši u proljeće.