fall off:To become less; decrease (變少；降低)
例如：Stock prices have fallen off.The number of staff meetings fell off after a few months.（股票價(jià)格下降了.工作人員的會(huì)議在幾個(gè)月后減少了.）
fall down:To fail to meet expectations；lag in performance（失?。何茨苓_(dá)到預(yù)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(dá)到預(yù)想目標(biāo).
fall into:介入,開(kāi)始.
usually you have just met,you start having a conversation or discussion with them（素不相識(shí),一見(jiàn)面就能進(jìn)入聊天或討論某個(gè)問(wèn)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(shí),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(shū)分三部分.