Mozak radi brzo.

• mozak = a noun (brain)
• radi = a verb form, from raditi
• brzo = an adverb (quickly / fast)

So the structure is:

subject + verb + adverb

Radi is the 3rd person singular present tense of raditi.

That means it is used with subjects like:

• on = he
• ona = she
• ono = it
• any singular noun, such as mozak

So mozak radi literally means the brain works / is working.

Because mozak is the clear subject of the sentence, and raditi here is used normally: to work / function / operate.

Radi se is a different construction and often means something more like:

• it is being done
• it is about... in some expressions

In Mozak radi brzo, the meaning is simply that the brain is functioning quickly, so plain radi is correct.

Mozak is in the nominative singular, because it is the subject of the sentence.

The basic dictionary form is also mozak.

A learner may notice that Serbian changes noun endings in other cases, but here no change is needed because the noun is the subject.

Because brzo is an adverb, while brz is an adjective.

• brz = fast (describes a noun)
• brzo = fast / quickly (describes a verb)

Here it describes radi (works), so Serbian uses the adverb:

• Mozak radi brzo. = The brain works quickly.

If you used brz, that would need to describe a noun, for example:

• brz mozak = a fast brain (less natural in many contexts, but grammatically it shows adjective use)

Yes. In English, fast can be both an adjective and an adverb. Serbian usually keeps those roles more separate:

• brz = adjective
• brzo = adverb

So brzo can correspond to English quickly or fast, depending on the sentence.

In this example, a natural translation could be either:

• The brain works quickly.
• The brain works fast.

Not completely. Serbian word order is more flexible than English word order.

Mozak radi brzo is correct, but you may also hear:

• Mozak brzo radi.

Both are natural. The difference is usually about emphasis or rhythm, not basic meaning.

Very roughly:

• Mozak radi brzo. = neutral, with brzo placed at the end
• Mozak brzo radi. = also very natural, with the adverb closer to the verb

A learner should understand both.

Because Serbian has no articles like English a and the.

So mozak can mean:

• a brain
• the brain
• brain

The exact meaning depends on context.

In this sentence, English usually translates it as The brain works fast/quickly, but Serbian does not need a separate word for the.

A simple English-friendly approximation is:

MO-zak RA-di BR-zo

A few useful notes:

• r in Serbian is a rolled or tapped r
• z sounds like the z in zoo
• brzo begins with the cluster brz-, which may feel difficult at first for English speakers

If you say it slowly:

mo-zak ra-di br-zo

that can help.

It can be written in both Serbian scripts.

• Latin: Mozak radi brzo.
• Cyrillic: Мозак ради брзо.

Both are standard Serbian.

No. Here radi means works / functions / operates.

Serbian raditi is a very common verb and can mean different things depending on context, including:

• to work (to function)
• to work (to do work / have a job)
• to do / make in some contexts

In Mozak radi brzo, it clearly means the brain functions quickly.

Yes. It is short, grammatical, and natural.

It sounds like a general statement, for example in a context like:

• talking about someone who thinks quickly
• describing mental sharpness
• saying that the brain is active or functioning fast

Because Serbian often allows short subject-verb-adverb sentences, Mozak radi brzo works perfectly well on its own.