Learn Polish: Ryba płynie do rzeki. - The fish is swimming toward the river.

Płynie is a verb of motion that describes movement happening right now in one specific direction (toward the river). If you wanted to say the fish swims generally, habitually, or without a specific destination, you would use pływa.

Why does the sentence use rzeki instead of rzeka?

The preposition do (meaning to or toward) always requires the noun that follows it to take the Genitive case. The basic dictionary form is rzeka, which becomes rzeki in the Genitive case.

If I want to say the fish is swimming in the river, do I just change do to w?

You would change the preposition to w, but you also have to change the noun's case. To say in the river (showing location rather than motion toward a destination), you use w plus the Locative case, which makes it w rzece.

Is it possible to change the word order to Do rzeki płynie ryba?

Yes! Polish word order is very flexible. However, putting ryba at the end changes the emphasis. Ryba płynie do rzeki is a neutral statement. Do rzeki płynie ryba emphasizes the word at the end, meaning it is a fish (not a duck or a boat) that is swimming to the river.