Ja sada čitam knjigu, a ona pije čaj.

Yes. In Serbian, the verb ending already shows the subject, so čitam by itself means I read / I am reading.

That means ja is often optional. It is included here because the sentence is contrasting two people:

• Ja sada čitam knjigu, a ona pije čaj.

Using ja and ona makes the contrast clearer: I am reading, she is drinking tea.

A more natural sentence in everyday speech could also be:

• Sada čitam knjigu, a ona pije čaj.

Both are correct.

A often means something like and, but it has a special nuance. It usually connects two ideas that are parallel or contrasted.

In this sentence:

• Ja sada čitam knjigu, a ona pije čaj.

A suggests something like:

• I am reading a book, while she is drinking tea
• I am reading a book, and she, on the other hand, is drinking tea

So it is not exactly the same as simple i (and), and it is not as strong as ali (but).

A quick comparison:

• i = simple addition
• a = contrast or parallel comparison
• ali = stronger contradiction, but

Because knjigu is the accusative singular form of knjiga (book).

In Serbian, a direct object usually goes in the accusative case. Here, the thing being read is the book, so knjiga changes form:

• knjiga = nominative
• knjigu = accusative

So:

• Knjiga je na stolu. = The book is on the table.
  (knjiga is the subject)
• Čitam knjigu. = I am reading a book/the book.
  (knjigu is the direct object)

Because čaj is a masculine inanimate noun, and in Serbian, masculine inanimate nouns usually have the same form in the nominative and accusative singular.

So:

• čaj = nominative
• čaj = accusative

That is why:

• Čaj je vruć. = The tea is hot.
• Ona pije čaj. = She is drinking tea.

By contrast, knjiga is feminine, and feminine nouns often change in the accusative singular:

• knjiga → knjigu

Yes, sada (now) can move. Serbian word order is fairly flexible.

Here it appears as:

• Ja sada čitam knjigu

But you could also say:

• Sada čitam knjigu.
• Ja čitam sada knjigu.
• Čitam knjigu sada.

Some versions sound more natural than others depending on emphasis. The given sentence is very normal and neutral.

In general:

• placing sada before the verb is common
• moving it changes emphasis slightly, not the basic meaning

They are in the present tense.

Serbian does not have a separate tense exactly like the English present continuous (am reading, is drinking). The present tense can cover both:

• I read
• I am reading

So:

• čitam can mean I read or I am reading
• pije can mean she drinks or she is drinking

In this sentence, sada (now) makes it clear that the actions are happening right now, so the natural English translation is:

• I am reading a book now, and she is drinking tea.

Their dictionary forms are:

• čitati = to read
• piti = to drink

The forms in the sentence are present-tense forms:

• čitam = I read / I am reading
• pije = she drinks / she is drinking

A few present-tense forms:

čitati

• ja čitam
• ti čitaš
• on/ona/ono čita
• mi čitamo
• vi čitate
• oni/one/ona čitaju

piti

• ja pijem
• ti piješ
• on/ona/ono pije
• mi pijemo
• vi pijete
• oni/one/ona piju

Because Serbian has no articles.

English uses:

• a book
• the book
• tea
• the tea

Serbian usually does not use a separate word for that. So:

• knjiga / knjigu can mean a book or the book
• čaj can mean tea or the tea

The exact meaning depends on context.

So čitam knjigu could mean:

• I am reading a book
• I am reading the book

And ona pije čaj could mean:

• she is drinking tea
• she is drinking the tea

Yes, absolutely.

Since the verb endings already show the subject, Serbian often drops subject pronouns when they are not needed.

So this sentence could be:

• Sada čitam knjigu, a pije čaj.

However, that version sounds odd because the second clause has no clear subject stated, and there is a contrast between two different people. So in practice, you would usually at least keep ona:

• Sada čitam knjigu, a ona pije čaj.

You could also keep both pronouns for extra contrast:

• Ja sada čitam knjigu, a ona pije čaj.

So yes, pronouns can be omitted, but whether they should be depends on clarity and emphasis.

Yes, normally yes.

In Serbian, when a joins two full clauses, a comma is usually written before it:

• Ja sada čitam knjigu, a ona pije čaj.

This is standard punctuation.

The sentence has two coordinated clauses:

1. Ja sada čitam knjigu
2. ona pije čaj

Because they are linked by a, the comma is expected.

Yes. čitati and piti are imperfective here.

That matters because imperfective verbs are normally used for:

• ongoing actions
• repeated actions
• general activities

Since the sentence describes what is happening now, imperfective is exactly what you would expect:

• Ja sada čitam knjigu = an ongoing action
• ona pije čaj = an ongoing action

A perfective verb would usually not be used the same way for a simple action in progress right now.

Č is pronounced roughly like ch in church, but usually a bit firmer.

So:

• čitam sounds roughly like CHEE-tam
• čaj sounds roughly like chai

A useful detail for learners: Serbian distinguishes č and ć, and English speakers often mix them up. In this sentence, both words use č, not ć.

Yes. In everyday Serbian, sad is a very common shorter form of sada.

So you can say:

• Ja sad čitam knjigu, a ona pije čaj.

This is very natural in speech.

In general:

• sada = a bit fuller, sometimes slightly more formal or careful
• sad = very common in normal conversation

Both mean now.