Przenoska zaczyna się przed bystrzem.

Przenoska is the subject because it is the thing that starts.

It is a feminine singular noun, which is why the verb is in the 3rd person singular form: zaczyna.

A useful pattern to notice:

• ta przenoska = this portage/carrying place
• przenoska zaczyna się = the portage begins / starts

Because Polish nouns have grammatical gender, that gender affects agreement in other parts of the sentence.

In Polish, zaczynać się / zacząć się is the normal way to say that something begins.

So:

• zaczynać = to begin/start something
• zaczynać się = to begin/start

Compare:

• Zaczynam lekcję. = I am starting the lesson.
• Lekcja zaczyna się o ósmej. = The lesson starts at eight.

In your sentence, przenoska is not starting something else; it itself begins at a certain place. That is why się is used.

Się is a very common Polish clitic often used in reflexive or middle-style verbs.

Here it does not literally mean itself in a strong English sense. In this sentence, it is just part of the standard verb expression:

• zaczynać się = to begin
• kończyć się = to end
• znajdować się = to be located

So it is best to learn zaczynać się as a whole expression.

Bystrzem is in the instrumental singular.

That happens because the preposition przed takes:

• instrumental when it means in front of / before in a static location sense
• accusative when it means movement to a position in front of something

Here the meaning is location, not movement, so Polish uses the instrumental:

• przed bystrzem = before/in front of the rapid

So the base noun is bystrze, and its instrumental singular form is bystrzem.

The dictionary form is bystrze.

It is a neuter noun. Its forms include:

• bystrze = nominative singular
• bystrza = genitive singular
• bystrzu = locative singular
• bystrzem = instrumental singular

So after przed, you get bystrzem, not bystrze.

Here przed is about place, not time.

In English, before can be temporal or spatial. Polish przed works similarly:

• przed obiadem = before lunch (time)
• przed domem = in front of the house (place)

In this sentence, it means that the starting point is located before the rapid in space.

A rough English-friendly guide:

pshe-NO-ska za-CHI-na shye pshet BISS-tshem

A few helpful sound notes:

• rz often sounds like zh or merges with ż
• cz sounds like ch in chop
• sz sounds like sh
• się is roughly shye
• przed is pronounced roughly pshet
• bystrzem has a cluster: bys-trzem

This is only approximate, but it can help at first.

Yes, Polish word order is more flexible than English, because case endings show grammatical relationships.

The neutral order here is:

• Przenoska zaczyna się przed bystrzem.

But other orders may be possible for emphasis, for example:

• Przed bystrzem zaczyna się przenoska.

That version puts more focus on the location first, almost like It is before the rapid that the portage begins.

Still, the original sentence is the most straightforward neutral wording.

Because the subject przenoska is singular.

Polish verbs agree with the subject in number:

• przenoska zaczyna się = singular
• przenoski zaczynają się = plural

So zaczyna matches one przenoska.

Zaczyna się comes from zaczynać się, which is imperfective.

Its perfective partner is zacznie się from zacząć się.

Very roughly:

• zaczyna się = is beginning / begins
• zacznie się = will begin / begins at one definite future point

In a simple descriptive sentence like this, zaczyna się is natural because it states where something begins in a general way.

Polish has no articles like a or the.

So Przenoska can mean:

• a portage
• the portage

Which one is meant depends on context.

That is completely normal in Polish, and learners need to get used to meaning being carried by context rather than articles.

Not really. That would sound unnatural in English.

Even though się often looks like itself, here it is just part of the Polish verb expression zaczynać się.

So a natural understanding is simply:

• The portage begins before the rapid.
• The portage starts before the rapid.

This is a good example of why Polish reflexive-style verbs often should not be translated literally word for word.