It sounds simple, but doing it is much easier said than done.
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From Dongfang Shuo's Han-dynasty 《非有先生论》 (also in 《汉书·东方朔传》): 於戏！可乎哉？可乎哉？谈何容易。 Dongfang Shuo sighs about how dangerous it is to speak frankly to a ruler.
usage
Usually a sentence-final exclamation rebutting someone's casual suggestion, or follows a statement implying 'sounds simple'. Often preceded by 说起来 / 做起来 / 要…谈何容易 for the classic contrast.
