Mielizna jest przed bystrzem.

• mielizna = shoal, sandbank, or shallow water
• jest = is
• przed = in front of / before
• bystrzem = rapid in the correct grammatical form after przed

So the sentence literally matches Shoal is before rapid, which in natural English would be something like The shoal is before the rapid or There is a shoal before the rapid.

Because przed requires the instrumental case when it describes a fixed location.

Here:

• dictionary form: bystrze = rapid
• after przed for location: przed bystrzem = in front of the rapid / before the rapid

This is a very common Polish pattern:

• przed domem = in front of the house
• przed szkołą = in front of the school
• przed bystrzem = before the rapid

So bystrzem is the instrumental singular form of bystrze.

Bystrze is neuter.

That matters because the case ending depends partly on the noun’s gender and declension pattern. For this noun:

• nominative singular: bystrze
• instrumental singular: bystrzem

So if you want to say before the rapid, you need przed bystrzem.

Mielizna is in the nominative singular.

It is the subject of the sentence, so it stays in its basic dictionary form:

• mielizna = nominative singular

Also, mielizna is a feminine noun.

In the present tense, Polish often omits jest in some kinds of sentences, especially where the meaning is still clear. But with a sentence like this, jest sounds normal and natural.

• Mielizna jest przed bystrzem. = fully explicit
• Mielizna przed bystrzem. = possible in some contexts, but more elliptical and less neutral

For learners, using jest here is the safest choice.

Yes. Polish word order is much more flexible than English word order.

For example, these are possible:

• Mielizna jest przed bystrzem.
• Przed bystrzem jest mielizna.

Both mean essentially the same thing, but the focus changes slightly.

• Mielizna jest przed bystrzem puts mielizna first, so it feels like you are talking about the shoal and saying where it is.
• Przed bystrzem jest mielizna puts the location first, which can sound more like Before the rapid, there is a shoal.

Because Polish has no articles.

English distinguishes:

• a shoal
• the shoal

Polish usually just says:

• mielizna

The exact meaning depends on context. So Mielizna jest przed bystrzem could correspond to:

• A shoal is before the rapid
• The shoal is before the rapid
• There is a shoal before the rapid

Context tells you which one is meant.

Here przed means before in a spatial sense: in front of or upstream of / before reaching something, depending on context.

It can also be used in a time sense:

• przed obiadem = before lunch
• przed snem = before sleep / before going to sleep

In this sentence, it is clearly spatial.

Not always. It most commonly takes:

• instrumental for location

  • przed bystrzem = in front of / before the rapid

• accusative for motion toward a position in front of something

  • wyjść przed dom = to go out in front of the house

So in your sentence, there is no movement; it describes where something is. That is why the instrumental is used.

Grammatically, yes. But the vocabulary is somewhat specialized or geographical.

• mielizna and bystrze are words you might meet in:
  • descriptions of rivers
  • navigation
  • geography
  • literature

So the sentence is natural Polish, but it is not the kind of sentence people say in ordinary daily conversation unless they are talking about water, boating, or terrain.

A rough guide:

• mielizna ≈ myeh-LEEZ-na
• jest ≈ yest
• przed ≈ pshet
• bystrzem ≈ BIS-tshem

A few useful sound notes:

• j in Polish sounds like English y in yes
• rz usually sounds like zh / sh-like voiced sound, but after p in przed, it is pronounced more like sh
• sz sounds like English sh

So the whole sentence is roughly: myeh-LEEZ-na yest pshet BIS-tshem

Yes, depending on context, that can be a very natural English translation.

Polish often uses a simple X jest przed Y structure where English might prefer either:

• X is before Y or
• There is X before Y

Because Polish has no articles, the sentence itself does not force one single English version. If the original meaning shown to you uses There is a shoal before the rapid, that is perfectly reasonable.