Voda u rijeci je duboka.

• Voda – water; noun, feminine, nominative singular. It is the subject of the sentence.
• u – preposition in. With a static location, it usually takes the locative case.
• rijeci – from rijeka (river); here it is locative singular (in the river), governed by u.
• je – is; 3rd person singular present of biti (to be), the copular verb.
• duboka – deep; adjective, feminine, nominative singular, agreeing with the subject voda.

The literal structure is: Water in (the) river is deep.

In Croatian, the preposition u (in) can take different cases, depending on meaning:

• Locative = location (where something is)
  
  • u rijeci = in the river (static location)
• Accusative = direction (where something is going)
  
  • u rijeku = into the river (movement into)

In Voda u rijeci je duboka, we are talking about water located in the river, not movement, so rijeci is in the locative case: u rijeci = in the river.

Rijeka is nominative (subject form), and cannot be used after u in this meaning.

Adjectives in Croatian must agree with the noun in gender, number, and case.

• voda – feminine, singular, nominative
• So duboka is the form of dubok (deep) that matches:
  • feminine
  • singular
  • nominative

Other forms:

• dubok – masculine singular nominative
• duboka – feminine singular nominative
• duboko – neuter singular nominative

Because voda is feminine, you must say duboka:
Voda je duboka. = The water is deep.

Standard Croatian uses a copular verb (a form of biti – to be) in such sentences.

• Correct standard form: Voda u rijeci je duboka.
• Dropping je: Voda u rijeci duboka. – sounds colloquial or telegraphic, not standard written Croatian.

In everyday speech, some people do omit je in short, informal statements (similar to note-taking style), but for correct usage, especially in writing or formal speech, keep je:
Voda u rijeci je duboka.

Croatian does not have separate words for the and a/an like English. Nouns are typically used without articles, and definiteness (the vs a) is understood from:

• Context
• Word order
• Situation

So voda can mean:

• the water (specific)
• water (in general)

In this sentence, Voda u rijeci je duboka is naturally understood as The water in the river is deep, because we are talking about a specific river that is contextually known or visible.

Voda is in the nominative singular.

• The nominative case is used mainly for the subject of the sentence.
• Here, voda (water) is what we are talking about; it is doing the “being deep”.

Simplified declension of voda (singular):

• Nominative: voda – water (as subject)
• Genitive: vode – of water
• Dative: vodi – to/for water
• Accusative: vodu – water (object)
• Locative: vodi – in/at water
• Instrumental: vodom – with water

In Voda u rijeci je duboka, voda is the subject, so nominative is required.

Both u and na can roughly translate as in / on / at, but they are used differently:

• u – for being inside or within something:
  • u rijeci = in the river (inside the body of water)
• na – for being on a surface or a more open/flat place:
  • na rijeci = literally on the river (e.g., on a boat on the river)

In this sentence, we mean the water that is contained within the riverbed, so u rijeci is correct: Voda u rijeci je duboka.

Yes, Croatian word order is more flexible than English, though the basic neutral order here is:

• Voda u rijeci je duboka. – neutral: The water in the river is deep.

Other possibilities:

• U rijeci je voda duboka. – emphasizes in the river, as a location where the water is deep (contrast: maybe in the lake it is not).
• Voda je duboka u rijeci. – slightly emphasizes deep in the river (as opposed to somewhere else).

All are grammatically correct; changing the order mainly affects emphasis and focus, not the core meaning.

• Voda u rijeci je duboka. – literally The water in the river is deep.

  • Focuses on the water depth (how deep the water is).

• Rijeka je duboka. – The river is deep.

  • A bit more general; often still understood as talking about the water depth, but grammatically the river is the subject, not the water.

Both can describe how deep the water is, but:

• First: highlights the water as the thing that is deep.
• Second: treats the river as a whole as deep.

You could say:

• Riječna voda je duboka.

Here:

• riječna – adjective meaning river (as in river water, water belonging to the river)
• voda – water
• je – is
• duboka – deep (feminine, agreeing with voda)

Difference in nuance:

• Voda u rijeci je duboka. – literally The water in the river is deep (location phrase).
• Riječna voda je duboka. – River water is deep (treats it as a type of water).

You need to put both nouns, the verb, and the adjective in the plural:

• Vode u rijekama su duboke.

Breakdown:

• Vode – plural of voda (waters); feminine nominative plural (subject).
• u rijekama – rijekama is locative plural of rijeka (in the rivers).
• su – plural of je; 3rd person plural of biti (they are).
• duboke – feminine nominative plural, agreeing with vode.

Structure: Vode (waters) u rijekama (in the rivers) su (are) duboke (deep).

Yes, in normal speech rijeci (locative/dative of rijeka – river) and riječi (plural of riječ – word) are pronounced the same.

• u rijeci – in the river
• ove riječi – these words

They differ only in spelling and meaning, not in everyday pronunciation. In your sentence, context (plus the form u) clearly shows that rijeci comes from rijeka (river).