Ona brzo čita poruku.

Yes. Serbian often drops subject pronouns because the verb form already shows the person and number.

• čita already means he/she/it reads / is reading
• So Čita poruku. is a perfectly natural sentence

Using ona can:

• make the subject clearer
• add contrast or emphasis, like she (not someone else)
• help when the context does not already tell you who is doing the action

So both Čita poruku. and Ona brzo čita poruku. are possible.

Čita is the 3rd person singular present tense of the verb čitati.

That means it matches:

• on = he
• ona = she
• ono = it

Some present-tense forms of čitati are:

• ja čitam = I read / am reading
• ti čitaš = you read / are reading
• on/ona/ono čita = he/she/it reads / is reading
• mi čitamo = we read / are reading
• vi čitate = you read / are reading
• oni/one/ona čitaju = they read / are reading

Because poruku is the accusative singular form of poruka.

• poruka = nominative singular, the basic dictionary form
• poruku = accusative singular, used here because it is the direct object of the verb čita

She is reading what?
→ poruku

This is a very common pattern for feminine nouns ending in -a:

• nominative: knjiga

• accusative: knjigu

• nominative: poruka
• accusative: poruku

Because brzo is an adverb, and it describes how she reads.

• brz / brza / brzo can be adjective forms meaning fast
• brzo is also the usual adverb meaning quickly / fast

Here it modifies the verb čita, not a noun, so the adverb is needed:

• brza poruka = a fast message — this does not make much sense in most contexts
• brzo čita = she reads quickly

So:

• adjectives describe nouns
• adverbs describe verbs, adjectives, or other adverbs

It can mean either, depending on context.

The Serbian present tense often covers both:

• she reads
• she is reading

So Ona brzo čita poruku. could describe:

• a general habit
• something happening right now

Context tells you which meaning is intended.

For example:

• as a habit: Ona brzo čita poruke. = she reads messages quickly
• right now: Pogledaj, ona brzo čita poruku. = look, she is reading the message quickly

Serbian word order is more flexible than English word order.

The neutral order here is:

• Ona brzo čita poruku.

But other orders are possible, for example:

• Brzo čita poruku.
• Poruku ona brzo čita.
• Ona poruku brzo čita.

These do not all sound equally neutral. Changing the order usually changes:

• emphasis
• focus
• style
• what sounds most natural in context

For a learner, Ona brzo čita poruku. is a very safe, natural basic order.

Because Serbian has no articles like English a/an and the.

So poruku can mean:

• a message
• the message

Which one is meant depends on context.

Serbian uses other tools instead, such as:

• context
• word order
• demonstratives like ovu, tu, onu
• possessives like njenu

Examples:

• Čita poruku. = she is reading a/the message
• Čita tu poruku. = she is reading that message
• Čita ovu poruku. = she is reading this message

This is a question of aspect, which is very important in Serbian.

• čitati = imperfective
  Focuses on the process, repetition, or ongoing action
• pročitati = perfective
  Focuses on completing the action, reading through to the end

So:

• Ona brzo čita poruku. = she reads / is reading the message quickly
• Ona će brzo pročitati poruku. = she will read the message quickly, all the way through / finish reading it quickly

In this sentence, čitati is used because it presents the action as ongoing or simply as an event in progress/habit, not specifically as completed.

č is pronounced roughly like ch in church, but usually a bit firmer and more consistent.

So:

• čita sounds approximately like CHEE-ta
• poruku sounds approximately like PO-roo-koo
• brzo sounds approximately like BR-zo
• ona sounds approximately like O-na

A useful thing about Serbian spelling is that it is very phonetic: words are usually pronounced much more like they are written than in English.

Yes. Serbian uses both Latin and Cyrillic scripts.

In Cyrillic, the sentence is:

Она брзо чита поруку.

Letter by letter:

• Ona → Она
• brzo → брзо
• čita → чита
• poruku → поруку

Both scripts are correct in Serbian.

They are:

• ona = she
• brzo = quickly / fast
• čitati = to read
• poruka = message

In the sentence, the actual forms are:

• ona = subject pronoun
• brzo = adverb
• čita = present tense form of čitati
• poruku = accusative singular of poruka

This is a good example of how Serbian words often change form depending on their grammatical role.