It's hard to change one's inherent nature or deeply ingrained habits, even if external circumstances change dramatically; similar to "a leopard cannot change its spots" or "old habits die hard."
大家都知道他习惯性迟到，因为山河易改，本性难移，很难指望他下次能准时。
Everyone knows he's habitually late, because mountains and rivers may change, but one's nature is hard to shift; it's difficult to expect him to be on time next.
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