If one had known the consequences today, why would they have acted that way in the first place? It expresses regret over past actions and implies that one should have thought more carefully.
小张看着自己当初随意做的决定导致了现在的困境，不禁感叹道：早知今日，何必当初啊！
Xiao Zhang looked at the difficult situation caused by his impulsive decision and sighed, 'If only I had known today, I wouldn't have acted that way in the first place!'
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