After learning "Ram wrote with a pen" ("se" = instrument), Nandini thinks "se" always shows the…

Why

Nandini took the rule from the previous example — 'से' means instrument — and reasonably tried applying it here too, the exact mirror image of Mohan's earlier mistake. Both errors are the same kind: locking 'से' to a single meaning and applying it everywhere without checking. In 'पेड़ से पत्ता गिरा', the tree isn't a tool used to do something — the leaf separated FROM the tree; here 'से' marks separation or origin, not instrument. Test it: 'दिल्ली से ट्रेन आई' shows the same separation/origin sense (the train departed from Delhi), not an instrument. Her brother is right: here 'से' shows separation or origin (अपादान कारक, apadan karak) — 'से' doesn't always mean the same thing; its meaning depends on how it's used in the sentence.