Zaštitna folija je na dušeku.

Because zaštitna is an adjective and it has to agree with the noun folija.

• folija is feminine singular
• so the adjective must also be feminine singular
• therefore: zaštitna folija

Compare:

• zaštitni dušek = a protective mattress (masculine)
• zaštitna folija = a protective film/sheet (feminine)
• zaštitno staklo = protective glass (neuter)

So this is a basic case of adjective-noun agreement in Serbian.

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

So:

• Zaštitna folija = protective film / protective cover
• je = is
• na dušeku = on the mattress

A useful thing to know is that je is a short form, called a clitic, and it usually appears near the beginning of the sentence, often in second position.

Because Serbian uses different cases after na depending on the meaning:

• na + locative = location, being on something
• na + accusative = movement, going onto something

Here the sentence describes a location:

• Zaštitna folija je na dušeku. = The protective film is on the mattress.

If you wanted movement, you would use:

• Stavio sam foliju na dušek. = I put the film onto the mattress.

So:

• na dušeku = on the mattress
• na dušek = onto the mattress

It is locative singular.

The basic form is:

• dušek = mattress

After na in a location meaning, Serbian uses the locative:

• na dušeku = on the mattress

For many masculine nouns, the locative singular ends in -u, which is what you see here.

Because na often works with two different cases:

1. Locative for position:

   • na stolu = on the table
   • na zidu = on the wall
   • na dušeku = on the mattress

2. Accusative for direction:

   • na sto = onto the table
   • na zid = onto the wall
   • na dušek = onto the mattress

This is a very common pattern in Serbian, and it is similar to the difference between English on and onto.

Yes. Serbian word order is more flexible than English word order.

The neutral version is:

• Zaštitna folija je na dušeku.

But you could also say:

• Na dušeku je zaštitna folija.
• Na dušeku je folija.

The basic meaning stays the same, but the emphasis changes.

For example:

• Zaštitna folija je na dušeku. = neutral statement
• Na dušeku je zaštitna folija. = emphasis on where it is

So Serbian often uses word order to highlight new or important information.

No. Serbian does not have articles like English a and the.

So Zaštitna folija je na dušeku could mean:

• The protective film is on the mattress
• A protective film is on the mattress

Which one sounds right depends on the context.

Serbian usually leaves this to the situation, previous conversation, or emphasis.

Not always. Folija is a broader word than English foil.

It can mean things like:

• film
• sheet
• plastic covering
• foil, depending on context

So zaštitna folija often means:

• protective film
• protective sheet
• protective cover layer

In this sentence, it probably refers to some kind of protective covering on the mattress, not necessarily metal foil.

A rough pronunciation guide:

• dušeku ≈ DOO-sheh-koo
• zaštitna ≈ ZAHSH-teet-nah
• folija ≈ FOH-lee-yah

A few sound notes:

• š sounds like English sh
• ž sounds like the s in measure
• j sounds like English y

So:

• dušek has š = sh
• folija has j = y

Usually, no, not in a normal standard sentence like this.

• Zaštitna folija je na dušeku. = correct, standard
• Zaštitna folija na dušeku. = sounds incomplete unless it is a note, label, or headline

Serbian sometimes drops parts of to be in very informal speech or in special styles, but for learners it is best to keep je here.

Yes, very normal.

Serbian commonly forms phrases with:

• adjective + noun

So:

• zaštitna folija = protective film
• plastična flaša = plastic bottle
• beli sto = white table

This is the standard and natural order. The adjective usually comes before the noun, just like in English.