當初

at that time / originally

to be / to act as / manage / withstand / when / during / ought / should / match equally / equal / same / obstruct / just at (a time or place) / on the spot / right / just at

at or in the very same... / suitable / adequate / fitting / proper / to replace / to regard as / to think / to pawn / (coll.) to fail (a student)

at first / (at the) beginning / first / junior / basic

to regret one's past deeds (idiom)

see 早知今日何必當初|早知今日何必当初[zao3 zhi1 jin1 ri4 he2 bi4 dang1 chu1]

if I (you, she, he...) had known it would come to this, I (you, she, he...) would not have acted thus (idiom) / to regret vainly one's past behavior