U ovoj rečenici glagol je naglašen.

Here u means in, so it expresses location, not movement. With that meaning, u normally takes the locative case in Croatian.

So:

• u ovoj rečenici = in this sentence

Compare:

• u ovoj rečenici = location, in this sentence
• u ovu rečenicu = movement, into this sentence or into this wording depending on context

It is locative singular.

Why?

• rečenica is a feminine singular noun
• after u meaning in, Croatian uses the locative
• the demonstrative ovaj must match the noun in gender, number, and case

So:

• ova rečenica = this sentence, nominative
• u ovoj rečenici = in this sentence, locative

Because ovaj changes form depending on the noun it goes with. In this sentence it modifies rečenici, so it has to agree with it.

Since rečenici is:

• feminine
• singular
• locative

the correct form is ovoj.

This is just normal Croatian agreement.

Because the base form rečenica is in the nominative, but after u meaning in, it must go into the locative singular.

So:

• rečenica = sentence
• u rečenici = in the sentence

The ending -i is a very common feminine singular ending for the dative/locative.

Because Croatian has no articles. There is no direct equivalent of English a/an/the.

So glagol can mean:

• a verb
• the verb

The exact meaning comes from context. In this sentence, English naturally uses the verb.

The subject is glagol.

It is in the nominative singular, and it is the thing being described as naglašen.

So structurally:

• glagol = subject
• je naglašen = predicate, is stressed/emphasized

Je is the 3rd person singular present form of biti, which means to be.

So here it means is.

It combines with naglašen to make:

• je naglašen = is stressed / is emphasized

It is a past passive participle used like an adjective.

In sentences like this, Croatian often uses that kind of form with biti:

• je naglašen = is stressed/emphasized
• je napisan = is written
• je otvoren = is open/opened

So grammatically it comes from a verb, but in this sentence it behaves adjectivally and agrees with the noun it describes.

Because it has to agree with glagol.

Glagol is:

• masculine
• singular

So the matching form is naglašen.

Compare:

• glagol je naglašen = the verb is stressed
• riječ je naglašena = the word is stressed
• slovo je naglašeno = the letter is stressed

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

This version starts with U ovoj rečenici to set the context first: in this sentence.

Other possible orders can be natural too, for example:

• Glagol je naglašen u ovoj rečenici.
• U ovoj je rečenici glagol naglašen.

The meaning stays very similar, but the emphasis or flow changes a little.

English usually has a more fixed order, but Croatian often moves parts around for style, emphasis, or information structure.

The most important ones here are:

• č in rečenici = like ch in church, but usually a bit firmer
• š in naglašen = sh as in ship
• j in ovoj and je = y as in yes
• c in rečenici = ts as in cats

A rough pronunciation guide:

• U ovoj rečenici glagol je naglašen
• approximately: oo oh-voy re-CHE-nee-tsee GLA-gol ye na-GLA-shen

That is only an approximation, but it helps with the main sounds.