Vodní proud je před jezem.

Because před changes the case of the noun that follows it.

In this sentence, před means in front of / before in a static location sense, so it takes the instrumental case:

• jez = nominative singular
• jezem = instrumental singular

So:

• před jezem = in front of the weir / before the weir

This is a very common Czech pattern:

• pod mostem = under the bridge
• před domem = in front of the house
• za školou = behind the school

Vodní proud is in the nominative case, because it is the subject of the sentence.

The sentence structure is:

• Vodní proud = the water current / stream current
• je = is
• před jezem = in front of / before the weir

So vodní proud is the thing being described, which is why it stays in the nominative.

Vodní is an adjective meaning water, watery, or related to water.

It comes from voda = water, but Czech often uses an adjective where English might use a noun as a modifier.

Compare:

• voda = water
• vodní proud = water current
• vodní nádrž = water reservoir
• vodní sporty = water sports

So vodní proud is a normal Czech way to say water current.

Proud means current, flow, or stream of water.

In this sentence, vodní proud means water current.

Be careful not to confuse Czech proud with the English word proud. They are unrelated in meaning.

Je is the 3rd person singular form of the verb být = to be.

So:

• já jsem = I am
• ty jsi = you are
• on/ona/ono je = he/she/it is

Since vodní proud is singular, Czech uses je:

• Vodní proud je... = The water current is...

Před usually means in front of, before, or prior to.

Its case depends on meaning:

• with the instrumental: location, no movement
  
  • před jezem = in front of / before the weir
• with the accusative: movement toward a position in front of something
  
  • postavit něco před jez = to place something in front of the weir

In your sentence, there is no movement, only location, so the instrumental is used: před jezem.

Yes. Czech word order is fairly flexible.

The neutral order here is:

• Vodní proud je před jezem.

But you could also say:

• Před jezem je vodní proud.

Both are grammatical. The difference is mainly in focus or emphasis.

• Vodní proud je před jezem.
  Neutral: talking about the current and saying where it is.
• Před jezem je vodní proud.
  Emphasis on the location: in front of the weir, there is a current.

English relies more on fixed word order; Czech uses word order more for information structure.

Czech does not have articles like English a/an and the.

So:

• Vodní proud can mean a water current or the water current
• jez can mean a weir or the weir

The exact meaning depends on context.

This is normal in Czech, and learners have to get used to understanding definiteness from the situation rather than from an article.

A rough pronunciation guide:

• Vodní ≈ VOD-nee
• proud ≈ prowd (with Czech ou, similar to the vowel sound in English go
  • glide, but not exactly English)
• je ≈ ye
• před ≈ pshyed or p-ryed approximately; the ě affects the pronunciation, and the ř sound is famously difficult for English speakers
• jezem ≈ YEH-zem

A few useful notes:

• í is a long vowel
• ě changes the sound of the consonant before it
• ř is a special Czech sound with no exact English equivalent
• stress in Czech is almost always on the first syllable of each word:
  • VODní
  • PROUD
  • JE
  • PŘED
  • JEzem

It can mean both, depending on context.

In Czech, před can express:

• physical position: in front of
• sequence/order: before

With something like a weir and a water current, English might translate it more naturally as:

• in front of the weir
• before the weir
• sometimes even upstream of the weir, depending on context

So the exact English wording may vary, even though the Czech is the same.

Czech adjectives agree with the nouns they describe in:

• gender
• number
• case

Here, proud is:

• masculine inanimate
• singular
• nominative

So the adjective also appears in the matching form:

• vodní proud

In this particular adjective, the form vodní is used for several genders in the singular nominative, so it may not look especially different yet. But the important point is that adjective agreement is always happening in Czech.

It is a real and correct Czech word, but for many learners it is not one of the first basic nouns they meet.

Jez means weir: a low barrier built across a river to control water flow.

So the sentence is grammatically simple, but one word in it may feel specialized if you do not already know river-related vocabulary.

Yes, slightly. It is grammatically correct, but out of context it may sound a little technical or descriptive, because both vodní proud and jez belong to physical/geographical or engineering vocabulary.

In real-life use, it would sound most natural in a context like:

• describing a river
• explaining water flow
• discussing safety near a weir
• reading a technical or environmental text

So if the sentence feels a bit unusual, that is more about the vocabulary and context than about the grammar.