fall down:To fail to meet expectations；lag in performance（失?。何茨苓_到預想；在成就上落后）
例如：That young boy fell down on the job.那小伙子工作未能達到預想目標.
fall into:介入,開始.
usually you have just met,you start having a conversation or discussion with them（素不相識,一見面就能進入聊天或討論某個問題）
例如：Over breakfast at my motel,I fell into conversation with the owner of a hardware shop.在 motel 用早餐時,我就同五金商店的店主聊天了.
還有‘分成,屬于’含意.
例如：The book falls into three parts.這本書分三部分.