Recimo, ugovor o najmu može biti dokaz.

Here Recimo is a discourse word meaning something like for example, let’s say, or suppose.

In this sentence, it introduces one possible example:

• Recimo, ugovor o najmu može biti dokaz.
• For example, a rental contract can be evidence.

It does not change the core grammar of the sentence. You could remove it and still have a complete sentence:

• Ugovor o najmu može biti dokaz.

Because Recimo is being used as a parenthetical introductory word, similar to English for example or say.

So the comma marks a small pause:

• Recimo, ...

This is very common in Croatian when words like recimo, na primjer, or similar expressions introduce an example.

Because Croatian normally connects ugovor with a prepositional phrase to show what the contract is about.

• ugovor = contract
• o najmu = about rental / concerning lease

So ugovor o najmu literally means contract about rental, which is the normal Croatian way to say rental agreement or lease contract.

Croatian often uses a structure like:

• knjiga o povijesti = a book about history
• razgovor o poslu = a conversation about work
• ugovor o najmu = a contract about renting

Because the preposition o requires the locative case.

The basic dictionary form is:

• najam = rental, lease

After o, it changes to the locative singular:

• o najmu = about the rental / about the lease

So:

• ugovor o najmu

is using najmu because of the preposition o.

Here ugovor is in the nominative singular because it is the subject of the sentence.

The structure is basically:

• ugovor o najmu = subject
• može biti = verb phrase
• dokaz = predicate noun

So the sentence means that the rental contract is the thing that can be something.

Može biti is made of:

• može = can, is able to
  (3rd person singular present of moći = to be able to, can)
• biti = to be

Together:

• može biti = can be

Examples:

• To može biti problem. = That can be a problem.
• On može biti umoran. = He can be tired / may be tired.
• Ugovor o najmu može biti dokaz. = A rental contract can be evidence.

Because Croatian has no articles like English a/an/the.

So:

• ugovor can mean a contract, the contract, or just contract, depending on context
• dokaz can mean proof, evidence, a proof, or a piece of evidence, depending on context

In this sentence:

• ugovor o najmu može biti dokaz

naturally means something like:

• A rental contract can be evidence
• The rental contract can be proof

English has to choose an article, but Croatian does not.

After biti (to be), Croatian often uses a noun in the nominative when it names or identifies what something is.

So:

• To je problem. = That is a problem.
• On je liječnik. = He is a doctor.
• Ugovor o najmu može biti dokaz. = A rental contract can be evidence.

Here dokaz is in the nominative singular.

It can mean either, depending on context.

In legal or administrative contexts, dokaz is often best understood as:

• evidence
• supporting proof
• documentary proof

So in this sentence, ugovor o najmu može biti dokaz means that a rental contract can serve as evidence or proof of something.

Yes, Croatian word order is fairly flexible, although some orders sound more natural than others.

The most neutral version here is:

• Ugovor o najmu može biti dokaz.

You might also hear:

• Dokaz može biti ugovor o najmu.

That shifts the emphasis a bit more toward dokaz.

And Recimo can also move around:

• Recimo, ugovor o najmu može biti dokaz.
• Ugovor o najmu, recimo, može biti dokaz.

All of these are possible, but the original sentence sounds very natural.

No. The correct standard noun is najam.

Its forms include:

• nominative: najam
• locative: najmu

So in the sentence, o najmu is the correct form.

Learners sometimes find najam → najmu surprising, but it is a normal case change.