Questions & Answers about Plan jest ważniejszy niż film.
With niż, the compared element stays in the same case as the thing it’s compared to. Here, plan is the subject in the nominative, so after niż you also use nominative: film.
- If you use od instead of niż, you switch to the genitive: od filmu.
- Filmem (instrumental) would be used in sentences like Plan jest filmem (The plan is a film), not in comparisons.
Yes: Plan jest ważniejszy od filmu is fully correct and very common in everyday speech. Differences:
- od + genitive (od filmu) is the default with nouns and pronouns.
- niż is also correct with nouns and keeps the same case as the first element (here nominative: niż film).
- Only niż works before full clauses and many adverbs: ważniejszy niż myślisz (more important than you think), lepiej niż wczoraj (better than yesterday).