Pas je iza vrata.

• Pas = the subject, in the nominative singular
• je = is, the 3rd person singular present of biti (to be)
• iza = the preposition behind
• vrata = door, used after iza in the required case

So the structure is basically:

subject + is + prepositional phrase

Because Serbian normally uses the verb biti (to be) in sentences like this.

So just as English says The dog is behind the door, Serbian says Pas je iza vrata.

Unlike some other Slavic languages, Serbian does not normally drop is in the present tense in standard speech. So Pas iza vrata would sound incomplete in ordinary Serbian.

Je is a clitic, which means it usually goes in second position in the clause.

In Pas je iza vrata, the first element is Pas, so je comes right after it.

You can also change the word order, but je still tends to stay in second position:

• Pas je iza vrata.
• Iza vrata je pas.

Both are possible, but the emphasis changes a little.

It is in the genitive.

The preposition iza requires the genitive case in standard Serbian:

• iza kuće = behind the house
• iza škole = behind the school
• iza vrata = behind the door

So after iza, you should expect a genitive form.

Because vrata is a plural-only noun in Serbian.

That means Serbian uses a grammatically plural form for something that English usually treats as singular:

• vrata = door / doors, depending on context

This is very common for learners to notice. Even when it means one door, Serbian still says vrata.

If you add adjectives, they also agree in the plural:

• otvorena vrata = an open door / open doors

So grammatically it behaves like a plural noun.

Because with vrata, the genitive plural looks the same as the basic form:

• nominative plural: vrata
• genitive plural: vrata

So after iza, the case changes, but the spelling stays the same.

That can be confusing at first, because you do not see an obvious ending change.

Serbian has no articles.

So Pas je iza vrata can mean:

• The dog is behind the door
• A dog is behind a/the door

The exact meaning depends on context.

If Serbian speakers want to be more specific, they can add other words, for example:

• taj pas = that dog
• neki pas = some/a certain dog

But very often no article-like word is used at all.

Yes. Serbian word order is fairly flexible.

For example:

• Pas je iza vrata. = neutral statement
• Iza vrata je pas. = puts more focus on behind the door
• Iza vrata pas je... = generally not natural in ordinary speech because of clitic placement

So the sentence can move around more than in English, but je still has to follow the normal clitic rules.

No. After iza, standard Serbian uses the genitive, not dative/instrumental/locative.

So:

• correct: iza vrata
• not correct here: iza vratima

Vratima is a different case form, but it is not the one required by iza.

No.

• vrata = door
• vrat = neck

They look similar, but they are different words.

That is another common beginner confusion, so it is worth memorizing them separately.