Ovo je prijenosni punjač.

Because ovo is being used on its own as a pronoun meaning this.

In Croatian, the ov- words change form depending on gender:

• ovaj = this (masculine)
• ova = this (feminine)
• ovo = this (neuter)

When you point to something and say This is ..., Croatian very often uses ovo as a neutral, standalone this:

• Ovo je ... = This is ...

If you put the noun directly after this, then the demonstrative must agree with the noun:

• ovaj punjač = this charger

So:

• Ovo je prijenosni punjač. = This is a portable charger.
• Ovaj punjač je prijenosan. = This charger is portable.

Je is a clitic in Croatian. Clitics are short, unstressed words that usually go in second position in the sentence or clause.

So in:

• Ovo je prijenosni punjač.

the first element is Ovo, and the clitic je comes right after it.

This is very normal Croatian word order. English speakers often expect something more rigid, but in Croatian this second-position rule is important.

You will see the same pattern in many sentences:

• Ovo je dobro.
• Ana je ovdje.
• Moj brat je liječnik.

This is a very common learner question.

Croatian masculine adjectives can have a long form and a short form in the nominative singular.

With this adjective:

• prijenosan = short form
• prijenosni = long form

Very roughly:

• the long form is commonly used before a noun
• the short form is commonly used as a predicate adjective, especially after je

So:

• prijenosni punjač = a portable charger
• Punjač je prijenosan. = The charger is portable.

That is why prijenosni sounds right in this sentence.

The adjective must agree with the noun in:

• gender
• number
• case

Here punjač is:

• masculine
• singular
• nominative

So the adjective also appears in the masculine singular nominative form:

• prijenosni punjač

This kind of agreement is one of the most important patterns in Croatian grammar.

A useful general rule is:

• nouns ending in a consonant are usually masculine

Since punjač ends in -č, it is normally treated as masculine.

That is why you get:

• prijenosni punjač
• ovaj punjač

not:

• prijenosna punjač
• ova punjač

Like all rules, there are exceptions in Croatian, but this pattern is very common and reliable.

It is in the nominative.

In an identification sentence with to be:

• Ovo je prijenosni punjač.

the noun phrase after je is normally in the nominative, because it names what something is.

So:

• prijenosni punjač = nominative singular

Also, ovo here is the neuter singular form used as the subject-like element.

No. Croatian does not have articles.

So punjač can mean:

• a charger
• the charger

depending on context.

In this sentence, English needs an article, but Croatian does not:

• Ovo je prijenosni punjač.

If Croatian wants to be more specific, it often uses word order, context, or demonstratives such as:

• ovaj punjač = this charger
• taj punjač = that charger

A rough English-friendly pronunciation is:

• OH-vo yeh pree-YEH-nos-nee POON-yach

A few useful sound notes:

• j sounds like y in yes
• č sounds like ch, but a bit firmer than in English
• nj sounds like ny, similar to the sound in canyon

So punjač is roughly POON-yach.

Yes, Croatian word order is flexible, but not all versions sound equally natural.

The neutral, standard sentence is:

• Ovo je prijenosni punjač.

Because je is a clitic, it still wants second position, so you cannot move words around completely freely.

Some reordered versions are possible only with special emphasis or contrast, but they may sound marked rather than neutral.

For a learner, the safest version is:

• Ovo je prijenosni punjač.

Yes, you could.

Both ovo and to can appear in identification sentences like this, but they are not exactly the same in feeling:

• ovo = this
• to = often that or sometimes a more neutral that/it

So:

• Ovo je prijenosni punjač. = This is a portable charger.
• To je prijenosni punjač. = That is a portable charger. or sometimes That’s a portable charger.

If you are physically showing something near you, ovo is usually the better choice.