Ovaj kaput je najjeftiniji u trgovini.

Because kaput is a masculine singular noun, and ovaj must agree with it.

In Croatian, words like this change form depending on gender, number, and case:

• ovaj kaput = this coat (masculine)
• ova jakna = this jacket (feminine)
• ovo odijelo = this suit/clothing item (neuter)

Here, kaput is masculine singular and is the subject of the sentence, so ovaj is the correct form.

Because kaput is in the nominative singular, which is the basic dictionary form and the usual form for the subject of a sentence.

In Ovaj kaput je..., ovaj kaput is the subject, so kaput stays in nominative singular.

Many masculine nouns in Croatian have no visible ending in the nominative singular, so kaput simply stays kaput.

Je is the 3rd person singular present form of biti (to be). It means is.

So:

• je = is

In this sentence, it links the subject ovaj kaput with the description najjeftiniji u trgovini.

It is the superlative form of the adjective jeftin (cheap).

The pattern is:

• jeftin = cheap
• jeftiniji = cheaper
• najjeftiniji = cheapest

A very common Croatian pattern is:

• comparative
  • prefix naj- = superlative

So najjeftiniji literally means something like most cheap, but in natural English it is the cheapest.

Because the adjective must agree with kaput.

Since kaput is:

• masculine
• singular
• nominative

the adjective also appears in the masculine singular nominative form:

• najjeftiniji

Compare:

• Ovaj kaput je najjeftiniji. = This coat is the cheapest.
• Ova jakna je najjeftinija. = This jacket is the cheapest.
• Ovo odijelo je najjeftinije. = This outfit is the cheapest.

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

English says the cheapest and the shop/store, but Croatian usually expresses that meaning through:

• context
• word order
• demonstratives like ovaj
• adjective forms and case endings

So najjeftiniji naturally corresponds to the cheapest, even though there is no separate word for the.

Because after u meaning in or inside, when you are talking about location, Croatian uses the locative case.

The noun trgovina changes like this:

• trgovina = nominative
• u trgovini = in the shop/store (locative)

So u trgovini means in the store/shop.

A useful contrast:

• u trgovini = in the store (location)
• u trgovinu = into the store (motion/destination)

Because trgovina is normally treated as an enclosed place, so Croatian uses u for in.

• u trgovini = in the shop/store

Croatian often uses na for things like:

• surfaces
• some public places or events
• certain idiomatic locations

But with trgovina, the standard expression is u trgovini.

The given order is the most neutral one:

• Ovaj kaput je najjeftiniji u trgovini.

Croatian word order is fairly flexible, so other versions are possible, for example:

• Ovaj je kaput najjeftiniji u trgovini.

This still means the same thing, but the emphasis may shift slightly.

One important detail: je is a clitic, so it usually cannot stand independently at the beginning of the sentence. It tends to come near the beginning, often after the first word or phrase.

It can mean both shop and store in English.

In everyday Croatian, trgovina is a general word for a retail place. Depending on context, you might translate it as:

• shop
• store

So u trgovini could be translated either as in the shop or in the store, depending on the style of English you want.