Learn Polish: Rzeczownik to ważne słowo, a czasownik to główne słowo. - A noun is an important word, and a verb is the main word.

In Polish, when you equate two nouns (Noun A is Noun B), you can use the particle to instead of the verb być (to be). It acts like an equals sign. So, Rzeczownik to słowo simply means "A noun is a word."

Does using to change the case of the words that follow it?

Actually, it prevents them from changing! The particle to connects two nouns in the Nominative case (the basic dictionary form). If you used jest instead, the second noun phrase would have to switch to the Instrumental case, making it Rzeczownik jest ważnym słowem. Using to lets you keep everything in the Nominative.

Why is the conjunction a used here instead of i for "and"?

The conjunction a is used to show a slight contrast or to compare two parallel things, often translating to "while" or "whereas" in English. Since we are comparing the roles of nouns and verbs, a is the most natural choice. The word i is used for simply grouping things together without contrast.

Do the grammatical terms rzeczownik and czasownik come from other common Polish words?