Questions & Answers about Moja siostra jest bardziej pracowita od ciebie.
Why do we use od ciebie to mean "than you"?
To make a comparison in Polish, you use the preposition od (which literally means "from") followed by the Genitive case. So, "than you" translates to od + ciebie (the Genitive form of the pronoun ty).
Can I use the shorter pronoun cię instead of ciebie here?
No, you must use ciebie. After prepositions like od, Polish requires you to use the long, stressed form of a pronoun. The short form cię is only used when the pronoun is standing on its own without a preposition.
Why use bardziej pracowita instead of adding a special comparative ending to the adjective?
Just like in English, Polish has two ways to say "more": changing the ending of the word (like "faster") or putting a word in front (like "more hardworking"). For longer adjectives with three or more syllables like pracowity, it is much more common and natural to just use bardziej (more) in front of the basic adjective.
Would bardziej pracowita change if the sentence was about a brother?