Learn Polish: Ten stolarz robi piękne stoły. - This carpenter makes beautiful tables.

It is derived from the noun stół (table). By taking the base root stol- and adding the suffix -arz (a very common suffix for professions), it literally translates to someone who makes or works with tables.

Why does the vowel change from ó in stół to o in stolarz?

The ó in stół occurs because the word ends in a consonant, creating a closed syllable. When you add the suffix -arz, you add a vowel which opens the syllable (sto-larz). When the syllable opens, the ó reverts to the base vowel o. You can see this exact same shift in the plural noun stoły.

Is there a rule for when to use -arz versus other profession suffixes like -ik or -awca?

While not perfectly predictable, -arz is typically added to nouns to describe a person who operates, produces, or works with that specific object (like stół to stolarz, or lek to lekarz). In contrast, suffixes like -awca usually attach to verbs to indicate the doer of an action.

Since piękne stoły is the direct object being made, why does it look exactly like the nominative case?