Why acted like that back then; expresses regret or reproach regarding past actions, often used in the phrase [[早知今日，何必当初|早知今日，何必當初|zao3 zhi1 jin1 ri4 he2 bi4 dang1 chu1]].

To regret bitterly what one did (or did not do) in the first place.

Used to express regret or to criticize someone by suggesting that if they had known it would come to this, they would not have acted as they did.

if I had only known it would come to this, I would not have acted as I did; used to express regret for past actions or to reproach someone for their current predicament

Used to express that if one had known things would turn out this way, one would not have acted as one did; to regret a past mistake in light of present consequences.