既然有今日的如此结果，何必在当初那么做。用以感慨现今的不如意，而深悔过去的错误。

[if there must be today,then why should there have been other days;if I had known it would come to this,Iwould have acted differently;if it has to be like this now,it's a pity it was even like that in the beginning] 既然有今日的如此结果,何必在当初那么做。用以感慨现今的不如意,而深悔过去的错误

宝玉在身后面叹道:“既有今日,何必当初。”——《红楼梦》

何必：

用反问的语气表示不必；既然不会下雨，何必带伞！

错误：

①不正确；与实际不合：错误的看法。

②指不正确的观点、行为等：犯错误｜错误非常严重。

今日：

今天：从上海来的参观团预定今日到达。

过去：

时间词，现在以前的时期（区别于‘现在、将来’）：过去的工作只不过像万里长征走完了第一步。