Staro doba je završeno.

Literally, staro doba means old era / old age / old period / old times, depending on context.

A very important point: it usually refers to a time period, not to a person's old age.
For a person's old age, Serbian more naturally uses starost.

So:

• staro doba = the old era / old times
• starost = old age as a stage of life

Because both words agree with doba, and doba is neuter singular.

In Serbian, words that describe a noun usually match it in gender and number.

Here:

• doba = neuter singular
• staro = neuter singular form of star
• završeno = neuter singular form of završen

So the sentence is internally consistent:

• staro doba = old era
• doba je završeno = the era is finished / over

Because Serbian has a small group of nouns that do not follow the most common ending pattern.

A learner often expects:

• nouns ending in -a -> feminine

But doba is an exception: it is neuter.
You can see that from agreement:

• ovo doba = this era
• staro doba = old era
• doba je završeno = the era is finished

So the safest approach is simply to learn doba as a neuter noun.

Završeno is the passive participle (or participle used adjectivally) of završiti = to finish / to complete.

With je, it forms a predicate meaning something like:

• is finished
• has been finished
• is over

In this sentence, it describes the state of doba:

• Staro doba je završeno.
  = The old era is over / has ended.

So although it comes from a verb, it behaves a lot like an adjective here and agrees with doba.

Because je is a clitic in Serbian. Clitics are short unstressed words that usually go in the second position of the sentence or clause.

So in:

• Staro doba je završeno

the first unit is Staro doba, and je comes right after it.

This is very normal Serbian word order. English speakers often expect the verb to stay in one fixed place, but Serbian is more flexible, especially with clitics.

Because Serbian has no articles. There is no direct equivalent of English the or a/an in ordinary grammar.

So staro doba can mean:

• the old era
• an old era
• old times

Context tells you which one is meant.

That is why Serbian sentences often look more bare or compact than English ones.

All three can work, but the most natural English choice depends on context.

For doba meaning an era or period, English often prefers:

• is over
• has ended

So this sentence will often sound more natural in English as:

• The old era is over.
• The old age has ended.

Is finished is possible, but with a time period it can sound slightly less natural than is over.

Yes, you could, and it would be understandable.

But there is a nuance:

• Staro doba je završeno = focuses on the resulting state: the old era is now finished/over
• Staro doba se završilo = focuses more on the event/process of ending

The version with je završeno can sound a bit more formal or literary.
The version with se završilo can sound more like it ended.

Yes. Serbian word order is flexible, but changing it changes the focus or style.

Neutral order:

• Staro doba je završeno.

More marked or literary:

• Završeno je staro doba.

Both are grammatical, but the first one is the most straightforward for a learner and the most neutral in tone.

It is in the nominative, because it is the subject of the sentence.

You can think of the structure like this:

• Staro doba = subject
• je završeno = predicate

So the sentence is basically:

• The old era
  • is finished / is over

That is why both staro and završeno appear in forms that match nominative neuter singular.