Kajakarz omija mieliznę i cumuje przy brzegu rzeki.

Because omijać takes the accusative case: you go around or avoid something.

• dictionary form: mielizna = a shoal, sandbank, shallow spot
• in the accusative singular: mieliznę

So:

• omija mieliznę = he/she is going around the shoal

This is a very common thing in Polish: the noun changes form depending on its role in the sentence.

Because the preposition przy requires the locative case.

• dictionary form: brzeg = bank, shore
• locative singular: brzegu

So:

• przy brzegu = at/by the bank

This is something worth memorizing with the preposition:

• przy + locative

Examples:

• przy stole = at the table
• przy oknie = by the window
• przy brzegu = by the shore/bank

Here rzeki is in the genitive case because it depends on brzeg:

• brzeg rzeki = the bank of the river

This is a very common Polish pattern:

• coś czegoś = something of something

Examples:

• drzwi domu = the door of the house
• kolor nieba = the color of the sky
• brzeg rzeki = the bank of the river

So in the full phrase:

• przy brzegu rzeki = by the riverbank

Cumować means to moor, dock, tie up a boat.

So cumuje means:

• he/she moors
• he/she is docking
• he/she ties up the boat

It is a boating-specific verb, so it fits well with kajakarz.

Both are 3rd person singular present tense forms:

• omija = he/she avoids / goes around
• cumuje = he/she moors / docks

The subject is kajakarz (the kayaker), which is singular, so both verbs must match that singular subject.

Polish often uses the present tense where English might translate with either:

• simple present: The kayaker goes around... and moors...
• present continuous: The kayaker is going around... and is mooring...

The exact English version depends on context.

Grammatically, kajakarz is a masculine noun, and in ordinary usage it usually means a male kayaker.

The feminine form is:

• kajakarka = female kayaker

However, in some contexts a masculine form may be used more generally, especially in dictionary-style examples. Still, if you specifically want to say a female kayaker, use kajakarka.

Polish has no articles like English a/an/the.

So:

• kajakarz can mean a kayaker or the kayaker
• mieliznę can mean a shoal or the shoal
• brzegu rzeki can mean the riverbank or a riverbank

The exact meaning is understood from context.

Yes, Polish word order is fairly flexible, because the case endings show the grammatical relationships.

The neutral order here is:

• Kajakarz omija mieliznę i cumuje przy brzegu rzeki.

But other orders are possible for emphasis, for example:

• Przy brzegu rzeki kajakarz cumuje po ominięciu mielizny.
• Mieliznę kajakarz omija i cumuje przy brzegu rzeki.

Still, the original version sounds natural and straightforward.

Both are imperfective verbs.

• omijać = to be going around / to avoid
• cumować = to be mooring / to moor repeatedly or generally

In a simple descriptive sentence like this, imperfective is natural because it presents the actions as part of an event or scene.

For learners, the key point is:

• imperfective often describes an action in progress, repeated action, or a plain statement of what happens

A rough English-friendly guide:

• Kajakarz ≈ ka-ya-kash
• omija ≈ o-MEE-ya
• mieliznę ≈ myeh-LEE-zneh
• i = ee
• cumuje ≈ tsoo-MOO-yeh
• przy ≈ pshih
• brzegu ≈ BZHEH-goo
• rzeki ≈ ZHEH-kee

A few important sound notes:

• rz usually sounds like the zh in measure
• cz sounds like ch in chop
• sz sounds like sh
• c sounds like ts
• y is not like English ee; it is a harder vowel

Stress in Polish is usually on the second-to-last syllable:

• ka-ja-KARZ
• o-MI-ja
• mie-LIZ-nę
• cu-MU-je
• RZE-ki

Because Polish often builds noun phrases differently from English.

Word-for-word, it is:

• przy = by/at
• brzegu = bank/shore
• rzeki = of the river

So literally:

• by the bank of the river

Natural English usually says:

• by the riverbank
• at the riverbank
• by the bank of the river

All of these are possible translations, but riverbank is often the most natural in English.