Questions & Answers about Mój plecak jest cięższy o sto gramów.
Why do we use o in this sentence, and what case follows it?
The preposition o followed by the Accusative case is used to express the margin of difference in comparisons. While in English you just say 'a hundred grams heavier', in Polish you literally say 'heavier by a hundred grams' (cięższy o sto gramów).
Is the numeral sto actually in the Accusative case here? It looks exactly like the Nominative.
Yes, it is in the Accusative case, which is required by the preposition o when discussing a margin of difference. For non-virile (non-masculine-personal) numbers like sto, the Accusative form is identical to the Nominative form.
Why is the noun gramów used instead of the basic form gram?
Gramów is the Genitive plural form of gram. As a quick refresher: in Polish, numerals of 5 and above (like sto) require the noun being counted to take the Genitive plural case.
Could I say bardziej ciężki instead of cięższy?
No, that would sound unnatural. For common, short adjectives like ciężki (heavy), Polish uses a single-word comparative form by modifying the stem and adding an ending (here, -szy). The word bardziej (more) is generally reserved for longer, more complex adjectives or participles.