On voli vodu.

Because vodu is the accusative singular form of voda.

In On voli vodu, the verb voleti / voleti (to love / like) takes a direct object, and direct objects in Serbian often go in the accusative case.

• voda = water (dictionary form, nominative)
• vodu = water (accusative, used here as the object)

So:

• voda = water as the subject
• vodu = water as the thing being liked/loved

Voli can mean both likes and loves, depending on context.

In Serbian, voleti is used for:

• loving a person
• liking food, drinks, activities, etc.

So On voli vodu could be understood as:

• He likes water
• or more literally He loves water

In everyday English, He likes water is usually the more natural translation.

Yes, Serbian often drops subject pronouns because the verb ending already tells you who the subject is.

So both of these are possible:

• On voli vodu.
• Voli vodu.

Since voli is the 3rd person singular form, it already means he/she/it likes/loves.
The pronoun on is included when you want to:

• emphasize he
• make the subject clearer
• contrast with someone else

For example:

• On voli vodu, a ona voli sok. = He likes water, and she likes juice.

In Serbian:

• on = he
• ona = she
• ono = it

So on clearly shows a masculine singular subject.

That is why On voli vodu means He likes/loves water.

The base form is voleti (sometimes also listed as voljeti in some regional standards, but voleti is standard Serbian).

Voli is the 3rd person singular present tense form.

A few present-tense forms are:

• ja volim = I like/love
• ti voliš = you like/love
• on/ona/ono voli = he/she/it likes/loves
• mi volimo = we like/love
• vi volite = you like/love
• oni/one/ona vole = they like/love

This is the neutral, standard word order:

Subject + Verb + Object

• On = subject
• voli = verb
• vodu = object

Serbian word order is more flexible than English because cases show grammatical function. So other orders are possible, such as:

• Vodu voli on.
• Voli on vodu.

But those usually add emphasis or sound more marked.
For a beginner, On voli vodu is the safest neutral pattern.

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

So voda or vodu can mean:

• water
• the water

The exact meaning depends on context.

In On voli vodu, English usually just says water, not the water, because it is talking about water in general.

A simple pronunciation guide is:

on VO-lee VO-doo

A bit more carefully:

• On sounds roughly like own but shorter and more open.
• voli = VO-lee
• vodu = VO-doo

A few helpful notes:

• Stress is usually on the first syllable here: VO-li, VO-du
• Serbian spelling is very phonetic, so words are usually pronounced as written.

Here vodu is being used as a mass noun, meaning the substance water in general.

So On voli vodu means he likes water as a drink or substance, not one specific unit of water.

This is similar to English:

• He likes water not
• He likes a water

If you wanted to refer to a specific water in context, Serbian could still use vodu, but the situation would make that clear.

Yes, in the right context it could.

Because Serbian has no article, vodu can mean:

• water
• the water

So depending on context, On voli vodu might mean:

• He likes water
• He loves the water

However, without special context, most learners should understand it as the more general idea: He likes water.

You keep vodu in the accusative, but change the subject and the verb form:

• Ja volim vodu. = I like water.
• Ti voliš vodu. = You like water.
• On voli vodu. = He likes water.
• Ona voli vodu. = She likes water.
• Mi volimo vodu. = We like water.
• Vi volite vodu. = You like water.
• Oni vole vodu. = They like water.

This is a good sentence pattern to memorize: [subject] + [form of voleti] + vodu