Przeszkoda jest między mielizną a bystrzem.

Because after między in this sentence, Polish uses the instrumental case.

The basic forms are:

• mielizna → instrumental: mielizną
• bystrze → instrumental: bystrzem

So the pattern is:

• między + instrumental
• między mielizną a bystrzem

This is very common in Polish when you say that something is located between two things.

Here, między takes the instrumental because the sentence describes a location, not movement.

So:

• między mielizną a bystrzem = between the shoal and the rapid/current area

A useful rule:

• location → usually instrumental
• movement into/between → sometimes accusative

For example, compare:

• Jest między drzewami. = It is between the trees.
• Wszedł między drzewa. = He went in among the trees.

In your sentence, nothing is moving, so instrumental is used.

After między, Polish normally uses a in the pattern:

• między X a Y

This corresponds to English between X and Y.

So:

• między mielizną a bystrzem

Here, a does not mean a contrast like but. In this structure, it simply links the two things being named after między.

Using i here would sound wrong in standard Polish.

Przeszkoda is the subject of the sentence, so it is in the nominative case.

Breakdown:

• przeszkoda = subject
• jest = verb, is
• między mielizną a bystrzem = prepositional phrase telling where the obstacle is

Also:

• przeszkoda is a feminine singular noun
• that is why the verb is jest in the singular

Jest is the 3rd person singular present tense of być (to be).

So:

• ja jestem = I am
• ty jesteś = you are
• on/ona/ono jest = he/she/it is

In this sentence, przeszkoda is singular, so Polish uses jest.

Yes, in normal standard Polish, jest is needed here.

This sentence is a regular statement of location:

• Przeszkoda jest między mielizną a bystrzem.

Polish sometimes omits forms of to be in very informal or special sentence types, but not normally in a sentence like this. Leaving out jest here would sound incomplete or unnatural.

Bystrze is a neuter noun.

Its nominative singular form is:

• bystrze

In the instrumental singular, many neuter nouns take the ending -em, so:

• bystrze → bystrzem

That is why the sentence has:

• między ... bystrzem

This is a normal declension pattern for many neuter nouns.

Because mielizna is a feminine noun, and in the instrumental singular, many feminine nouns ending in -a change to -ą.

So:

• mielizna → mielizną

This is a very common pattern:

• kobieta → kobietą
• rzeka → rzeką
• mielizna → mielizną

So the ending -ą is a strong clue that you are looking at the instrumental singular of a feminine noun.

Yes, Polish word order is fairly flexible.

The original sentence is perfectly natural:

• Przeszkoda jest między mielizną a bystrzem.

But you could also say:

• Między mielizną a bystrzem jest przeszkoda.

The basic meaning stays the same. The difference is mostly one of focus or emphasis:

• starting with Przeszkoda puts the obstacle first
• starting with Między mielizną a bystrzem highlights the location first

English is much more rigid about word order than Polish.

Because Polish does not have articles.

English needs words like:

• a
• an
• the

Polish usually expresses that kind of information through context instead.

So przeszkoda can mean:

• an obstacle
• the obstacle

depending on the situation.

The same is true for mielizna and bystrze.

A few sounds may stand out to an English speaker:

• sz = like sh in ship
• rz = usually like the zh sound in measure
• ą = a nasal vowel, often roughly like on/om depending on context
• y = not the English ee sound; it is a more central vowel
• cz would be like ch in chop, though it does not appear here

Very rough pronunciation help:

• Przeszkoda ≈ pshesh-KO-da
• między ≈ MYEN-dzy / MYEN-zhy depending on how narrowly you describe it
• mielizną ≈ mye-LIZ-non
• bystrzem ≈ BIS-tshem or BYS-tshem with a Polish y, not English i

These are only approximations, but they can help at the start.

It describes position.

The key clue is the structure:

• jest między ... = is between ...

Nothing in the sentence suggests movement. It simply tells you where the obstacle is located.

That is also why między is followed by the instrumental case here.