Mobilna mreža radi dobro.

• mobilna = mobile
• mreža = network
• radi = works / is working / functions
• dobro = well

So the sentence literally looks like Mobile network works well, and in natural English that is The mobile network works well.

Because mobilna has to agree with mreža.

Mreža is:

• feminine
• singular
• here in the nominative case, because it is the subject of the sentence

So the adjective must also be feminine singular nominative:

• masculine: mobilni
• feminine: mobilna
• neuter: mobilno

That is why you get mobilna mreža.

Yes, mreža is feminine.

A very common clue in Serbian is that many feminine nouns in the dictionary form end in -a, and mreža does. That pattern is not perfect for every noun in the language, but it is very common and useful.

Because mreža is feminine, words that describe it also take feminine forms, like mobilna.

That is a very common and important point.

The verb raditi can mean:

• to work in the sense of having a job
• to work / function for machines, devices, systems, services, etc.
• sometimes more generally to do / operate

So in this sentence, radi means functions / works properly.

Examples:

• Telefon radi. = The phone works.
• Internet ne radi. = The internet isn’t working.
• On radi u školi. = He works at a school.

So raditi is broader than just job-related work.

Because dobro here is an adverb, not an adjective.

It modifies the verb radi:

• radi kako? = works how?
• answer: dobro = well

Compare:

• dobra mreža = a good network
  Here dobra is an adjective describing the noun mreža.
• mreža radi dobro = the network works well
  Here dobro is an adverb describing the verb radi.

This is very similar to English good vs well:

• a good network
• works well

Radi is the:

• 3rd person singular
• present tense of raditi

So it matches the singular subject mobilna mreža.

A few present-tense forms of raditi are:

• ja radim = I work / am working
• ti radiš = you work
• on/ona/ono radi = he/she/it works
• mi radimo = we work
• vi radite = you work
• oni/one/ona rade = they work

Since mreža is a singular thing, Serbian uses radi.

Serbian normally does not use articles like English a and the.

So mobilna mreža can mean:

• a mobile network
• the mobile network

Which one is meant depends on context.

In a sentence like this, English usually needs the: The mobile network works well. Serbian does not.

Yes. Serbian word order is more flexible than English word order.

All of these are possible:

• Mobilna mreža radi dobro.
• Mobilna mreža dobro radi.

Both mean essentially the same thing: The mobile network works well.

The difference is mostly one of rhythm, emphasis, or style. In everyday speech, Mobilna mreža dobro radi may sound a bit more natural to some speakers, but Mobilna mreža radi dobro is also perfectly understandable.

The tricky part for English speakers is usually ž.

• ž is pronounced like the s in measure or the g in genre
• so mreža sounds roughly like MRE-zha

A rough pronunciation guide:

• mobilna ≈ MO-beel-na
• mreža ≈ MRE-zha
• radi ≈ RA-dee
• dobro ≈ DO-bro

Also, Serbian usually pronounces words quite consistently, much more regularly than English spelling does.

You can use other verbs, but raditi is very common and natural.

Some alternatives:

• Mobilna mreža funkcioniše dobro. = The mobile network functions well.
• Mobilna mreža radi odlično. = The mobile network works excellently.
• Mobilna mreža ne radi. = The mobile network isn’t working.

Raditi is often the most everyday, practical choice when talking about whether something is working properly.

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

The basic structure is:

• Mobilna mreža = subject
• radi = verb
• dobro = adverb

So this is a straightforward subject + verb + adverb sentence.

If mreža appeared in another role, its form might change depending on the case, but here it stays in the basic nominative form: mreža.