Ovo je rezervna baterija.

• Ovo = this
• je = is
• rezervna = spare / backup
• baterija = battery

So grammatically, it is:

• Ovo = subject-like pointing word: this
• je = the verb to be
• rezervna baterija = a spare battery

Because ovo here does not directly modify baterija.

In this sentence, ovo means something like this thing here. It stands on its own as a pronoun. In Croatian, that form is often neuter singular: ovo.

Compare:

• Ovo je baterija. = This is a battery.
• Ova baterija je nova. = This battery is new.

So:

• ovo = this (thing), standing alone
• ova baterija = this battery, directly attached to the noun

That is why ovo is correct here, even though baterija is feminine.

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

So:

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

In Ovo je rezervna baterija, je simply means is.

Because je is a clitic, a short unstressed form. In Croatian, clitics usually go in the second position in the sentence or clause.

So:

• Ovo je rezervna baterija. = natural
• putting je first would not sound right in a normal statement

This second-position rule is very common in Croatian, so you will see it often with words like je, se, ga, mu, and so on.

Croatian has no articles, so it does not use words like a, an, or the the way English does.

That means:

• baterija can mean a battery or the battery
• the exact meaning depends on context

So rezervna baterija can naturally mean a spare battery here.

Because adjectives in Croatian must agree with the noun they describe.

Baterija is:

• feminine
• singular
• nominative here

So the adjective must also be:

• feminine
• singular
• nominative

That gives:

• masculine: rezervni
• feminine: rezervna
• neuter: rezervno

Examples:

• rezervni ključ = spare key
• rezervna baterija = spare battery
• rezervno kolo = spare wheel

It is in the nominative singular.

In a sentence like Ovo je rezervna baterija, the noun after je is a predicate noun, and in this kind of simple identification sentence Croatian normally uses the nominative:

• Ovo je kuća.
• To je moj brat.
• Ovo je rezervna baterija.

So rezervna baterija is nominative singular.

Yes, it is a normal and natural phrase.

Depending on context, Croatian can also use words like:

• pričuvna baterija
• zamjenska baterija

But rezervna baterija is straightforward and easy to understand, and it is a good standard translation of spare battery.

Yes, very often you can.

Both ovo and to can appear in sentences like this:

• Ovo je rezervna baterija.
• To je rezervna baterija.

A rough difference is:

• ovo often feels more like this here
• to often feels more like that / that thing / this depending on context

In real conversation, both can be used for identifying something, and the exact choice can depend on what is being pointed out and how close it feels to the speaker.

Yes, Croatian word order is more flexible than English word order, but not every version sounds equally neutral.

The most neutral version is:

• Ovo je rezervna baterija.

You may also hear other orders for emphasis, but they are more marked, for example:

• Rezervna baterija je ovo. = This is the spare battery.
• Baterija je rezervna. = The battery is spare / backup.

For a beginner, Ovo je rezervna baterija is the best pattern to learn first.

You add nije = is not.

• Ovo nije rezervna baterija. = This is not a spare battery.

Here nije is the negative form of je.

You would say:

• Ovo su rezervne baterije. = These are spare batteries.

Changes:

• je becomes su = are
• rezervna becomes rezervne
• baterija becomes baterije

So the adjective and noun both change to plural.

A simple English-friendly approximation is:

OH-vo yeh reh-ZEHRV-na ba-teh-REE-ya

A few helpful points:

• j in Croatian is like English y
  • je sounds like yeh
• v is pronounced clearly
• rija in baterija sounds roughly like ree-ya

Croatian spelling is quite phonetic, so once you know the sound of each letter, pronunciation becomes much easier than in English.