Ovo je pištaljka.

Because ovo here is a standalone demonstrative pronoun, meaning this or this thing. It is not directly attached to the noun.

So the structure is:

• Ovo = this
• je = is
• pištaljka = whistle

That gives Ovo je pištaljka = This is a whistle.

If you wanted to say this whistle, then the demonstrative would have to agree with the feminine noun:

• ova pištaljka = this whistle

So:

• Ovo je pištaljka. = This is a whistle.
• Ova pištaljka... = This whistle...

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

So it literally works like is in English:

• Ovo je pištaljka. = This is a whistle.

It is a very common form, but notice that je is a clitic, which means it normally cannot stand alone and usually appears in the second position of the sentence.

Yes, pištaljka is a feminine singular noun.

A big clue is the ending -a. In Croatian, most nouns ending in -a are feminine.

That matters for agreement:

• ova pištaljka = this whistle
• mala pištaljka = a small whistle

So even though the sentence starts with ovo, the noun itself is still feminine. The reason is that ovo is not directly modifying the noun here.

Croatian does not have articles like English a and the.

So where English says:

• This is a whistle

Croatian simply says:

• Ovo je pištaljka.

Whether the noun is understood as a whistle or the whistle depends on context. Croatian usually relies on context, word order, and words like demonstratives or possessives instead of articles.

It is in the nominative singular.

In sentences like this, where you identify something with to be, Croatian normally uses the nominative:

• Ovo je knjiga. = This is a book.
• Ovo je kuća. = This is a house.
• Ovo je pištaljka. = This is a whistle.

So pištaljka is the basic dictionary form here.

A rough pronunciation guide is:

• pi = like pee
• š = sh
• talj = roughly taly, with a soft lj
• ka = ka

So an approximate English-friendly version is pee-SHTAL-yka.

Two letters are especially important:

• š = sh as in ship
• lj = a soft sound somewhat like ly in million, though not exactly the same in all accents

You do not pronounce lj as a strong English l plus j separately.

Croatian word order is fairly flexible, but Ovo je pištaljka is the neutral, standard order.

Because je is a clitic, it usually comes near the beginning, in second position. That makes this order especially natural:

• Ovo je pištaljka.

Other orders are possible in special contexts for emphasis, but they are not the basic version a learner should start with.

Yes, you can.

To je... is also very common in Croatian for identifying something:

• To je pištaljka.

The difference is roughly this:

• ovo often feels more like this here, something near the speaker or being shown
• to can mean that, but in many identification sentences it is also used very naturally where English might say this or it

So both can be correct, depending on context. If you are physically showing an object, Ovo je pištaljka is especially natural.

You would say:

• ova pištaljka

That is because now the demonstrative is directly describing the noun, so it must agree with pištaljka, which is feminine singular.

Compare:

• Ovo je pištaljka. = This is a whistle.
• Ova pištaljka = this whistle

This is a very important distinction in Croatian:

• ovo = independent this
• ova = this used with a feminine noun