Kirija i depozit su u ugovoru.

Because the subject is kirija i depozit — two things joined by i (and). In Serbian, a compound subject normally takes a plural verb, so you use su (are), not je (is).

• kirija je u ugovoru = The rent is in the contract
• depozit je u ugovoru = The deposit is in the contract
• kirija i depozit su u ugovoru = The rent and deposit are in the contract

Ugovoru is in the locative case.

The preposition u often means in or inside, and when it has that meaning, it usually requires the locative:

• ugovor = contract (dictionary form, nominative)
• u ugovoru = in the contract (locative)

So the ending changes from ugovor to ugovoru because of the preposition u.

Serbian does not have articles like English a/an and the.

So:

• kirija can mean rent or the rent
• depozit can mean deposit or the deposit
• ugovoru can mean the contract, depending on context

Serbian speakers understand definiteness from context, not from separate article words.

Yes. Serbian word order is more flexible than English.

The basic sentence:

• Kirija i depozit su u ugovoru.

But you could also hear:

• U ugovoru su kirija i depozit.

Both are grammatical. The difference is mostly about focus or emphasis.

• Kirija i depozit su u ugovoru focuses first on rent and deposit
• U ugovoru su kirija i depozit focuses first on where they are: in the contract

I is the conjunction and.

It connects the two nouns:

• kirija = rent
• depozit = deposit
• kirija i depozit = rent and deposit

It works very much like English and.

The nouns are:

• kirija — feminine
• depozit — masculine
• ugovor — masculine

Gender matters in Serbian because it affects agreement and endings. In this sentence, the most important agreement point is the verb:

• even though kirija is feminine and depozit is masculine,
• together they form a plural subject,
• so the verb is su.

Yes, kirija is a very common everyday word for rent.

You may also see more formal or legal words, such as:

• najamnina
• sometimes stanarina in certain contexts

But in normal speech, kirija is extremely common and natural.

They are the subject of the sentence, so they are in the nominative case, which is the basic dictionary form.

That is why you see:

• kirija
• depozit

The noun that changes is ugovor, because it comes after u and must go into the locative:

• ugovor → u ugovoru

Yes. U can take either the locative or the accusative, depending on meaning.

• u + locative = in / inside a place or state
  
  • u ugovoru = in the contract
• u + accusative = into something, usually movement toward the inside
  
  • example: Stavio sam papir u ugovor = I put the paper into the contract/file

In your sentence, there is no movement, just location, so locative is used.

It is understandable and grammatical, but in real-life Serbian a speaker might also say something a bit more explicit, especially in formal contexts:

• Kirija i depozit su navedeni u ugovoru. = The rent and deposit are stated in the contract.
• Kirija i depozit pišu u ugovoru. = more colloquial, roughly The rent and deposit are written in the contract

Your original sentence is still fine, especially if the point is simply that those items are included in the contract.

You would use je instead of su.

For example:

• Kirija je u ugovoru. = The rent is in the contract.
• Depozit je u ugovoru. = The deposit is in the contract.

So the pattern is:

• one thing → je
• two or more things → su