Ona sada pije mleko i sok.

Yes. Serbian often drops subject pronouns when the verb already makes the subject clear.

So:

• Ona sada pije mleko i sok.
• Sada pije mleko i sok.

Both can mean the same thing.

Including ona adds emphasis or clarity, for example if you want to stress that she is the one drinking, not someone else.

Because pije is the 3rd person singular present tense form of piti (to drink).

Here is the present tense of piti:

• ja pijem = I drink / am drinking
• ti piješ = you drink / are drinking
• on/ona/ono pije = he/she/it drinks / is drinking
• mi pijemo = we drink / are drinking
• vi pijete = you (plural/formal) drink / are drinking
• oni/one/ona piju = they drink / are drinking

Since the subject is ona (she), the correct form is pije.

It can mean both, depending on context.

Serbian present tense often covers both:

• she drinks
• she is drinking

In this sentence, because of sada (now), the meaning is clearly is drinking now.

So sada helps make the action feel ongoing at the present moment.

Yes, sada means now.

It tells you that the action is happening at this moment. Without it, the sentence could sound more general:

• Ona pije mleko i sok. = She drinks milk and juice / She is drinking milk and juice.
• Ona sada pije mleko i sok. = She is drinking milk and juice now.

A very similar word is sad, which also means now in everyday Serbian. In many contexts, sad and sada are interchangeable.

They are direct objects here, and in Serbian that usually means the accusative case. But sometimes the accusative looks exactly like the basic dictionary form.

That is what happens here:

• mleko is a neuter noun, and its nominative and accusative singular are the same: mleko
• sok is an inanimate masculine noun, and for inanimate masculine nouns, nominative and accusative singular are usually the same: sok

So the objects are in the accusative, but you do not see a change in form.

A good example is a feminine noun ending in -a.

For example:

• voda = water (nominative)
• pijem vodu = I am drinking water (accusative)

Here the object changes from voda to vodu.

So Serbian learners often need to remember that:

• some accusative forms change visibly
• some do not

In mleko i sok, both happen to stay the same in form.

Because Serbian does not have articles like English does.

So:

• mleko can mean milk, the milk, or sometimes some milk
• sok can mean juice, the juice, or some juice

Context tells you which meaning is intended.

This is very normal in Serbian, and learners usually get used to it quickly.

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

All of these are possible:

• Ona sada pije mleko i sok.
• Ona pije sada mleko i sok.
• Sada ona pije mleko i sok.
• Mleko i sok ona sada pije.

But they do not all sound equally neutral.

The most natural neutral version here is:

• Ona sada pije mleko i sok.

Changing the order usually changes emphasis or style rather than basic meaning.

Yes, i means and.

Here it connects the two objects:

• mleko i sok = milk and juice

It is a very common word and works much like English and.

Grammatically, it means she is drinking milk and juice. Serbian does not force you to decide whether that means literally at the same time, during the same sitting, or just as part of the current situation.

In normal interpretation, most people would understand that she is having both.

If you wanted to be more specific, Serbian could make that clearer with extra words.

This is just part of how the verb conjugates.

The infinitive is piti (to drink), but present-tense forms are built from a present stem that gives:

• pijem
• piješ
• pije
• pijemo
• pijete
• piju

So this is not random; it is simply the normal pattern of this verb.

A learner should memorize piti as an infinitive and then learn its present-tense forms separately.

Both exist, depending on the regional standard.

• mleko is standard in Serbian as used in Serbia
• mlijeko appears in some other South Slavic standards and dialects

If you are learning standard Serbian, mleko is the expected form.

A simple pronunciation guide is:

• Ona ≈ OH-nah
• sada ≈ SAH-dah
• pije ≈ PEE-yeh
• mleko ≈ MLEH-koh
• i ≈ ee
• sok ≈ sohk

A rough full pronunciation:

• OH-nah SAH-dah PEE-yeh MLEH-koh ee sohk

A few useful notes:

• Serbian spelling is very phonetic.
• j is pronounced like English y in yes.
• ije in pije is pronounced roughly yee-eh, not like English pie.

Yes.

• Ona pije mleko i sok. is completely correct.

But without sada, it is less specifically tied to right now. It could mean:

• she drinks milk and juice (habitually), or
• she is drinking milk and juice (in the current situation)

Adding sada makes the immediate time reference much clearer.

Yes, piti is imperfective.

That matters because the present tense of an imperfective verb can describe:

• an action happening now
• a habitual action
• a repeated action

In this sentence, sada tells you it is happening now.

Serbian also has a perfective partner, often popiti, which usually means to drink up / finish drinking. But perfective verbs normally do not describe an ongoing present action in the same way.

So for she is drinking now, pije from piti is exactly what you want.