Radno vreme je dobro.

Because radno is an adjective agreeing with vreme.

• The dictionary form is radni = working
• vreme = time, and it is neuter singular
• Adjectives in Serbian must agree with the noun in gender, number, and case

So:

• radni = masculine singular
• radna = feminine singular
• radno = neuter singular

Since vreme is neuter singular, you get radno vreme.

Literally, it means working time.

In real usage, it often means:

• working hours
• business hours
• office hours
• opening hours

So even though the Serbian phrase is grammatically singular, English often translates it with a plural word like hours.

For the same reason as radno: dobro agrees with the subject radno vreme, which is neuter singular.

Here dobro is a predicate adjective after je.

Compare:

• Posao je dobar. = The job is good.
  masculine singular
• Plata je dobra. = The salary is good.
  feminine singular
• Radno vreme je dobro. = The working hours are good.
  neuter singular

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

So:

• je = is

In standard Serbian, you normally need this verb in a sentence like this:

• Radno vreme je dobro.

Unlike some languages, Serbian does not usually drop the present-tense to be in normal sentences of this kind.

Grammatically, it is singular.

The head noun is vreme, which is singular, so the whole subject is singular:

• Radno vreme je... not Radno vreme su...

This can feel strange to English speakers because English often says working hours or business hours, which are plural. Serbian just uses a different structure.

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

Breakdown:

• radno = nominative singular neuter
• vreme = nominative singular neuter

The sentence structure is basically:

• subject
  • je
    • predicate adjective

So:

• Radno vreme = subject
• je = is
• dobro = good

There is no separate word for the because Serbian does not have articles like English.

So Radno vreme je dobro can mean:

• The working hours are good
• Working hours are good
• The business hours are good

Which one sounds best depends on context.

Yes, Serbian word order is more flexible than English, but the version you have is the most neutral and standard:

• Radno vreme je dobro.

You may also hear:

• Dobro je radno vreme.

That version is possible, but it sounds more marked or emphatic, as if you are highlighting good.

For learners, the safest default is the original order.

Yes. That is a regional/standard variation.

You may see:

• radno vreme in Ekavian
• radno vrijeme in Ijekavian

Both are normal depending on the variety of Serbian being used. The meaning and grammar stay the same.

The sentence is grammatical and can be natural in the right context, especially if you are evaluating someone’s schedule or a store’s hours.

However, Serbian speakers might also use more specific phrases depending on what they mean:

• Radno vreme mi odgovara. = The working hours suit me.
• Radno vreme nije loše. = The working hours are not bad.
• Imaju dobro radno vreme. = They have good business hours.

So Radno vreme je dobro is fine, but in conversation a more specific sentence may sound more idiomatic.

You replace je with nije:

• Radno vreme nije dobro. = The working hours are not good.

That is the normal way to negate this sentence.