What is the legal ruling for a husband who swore to his wife by prohibition and a triple divorce if she did a certain thing, and she forgot the oath and did that thing?

If a wife does what her husband forbade her from, and he attached her prohibition (making her forbidden to him) to it, but she did it forgetfully, then the more authoritative opinion is that he does not break his oath. However, if the husband conditioned the triple divorce on his wife doing something: - If he intended to condition the divorce on his wife having done this thing in the past, then he has broken his oath, and the triple divorce takes effect. - If he intended to condition the triple divorce on his wife doing the thing in the future, then the divorce does not take effect unless the wife does this thing, in which case the triple divorce takes effect. - Some scholars are of the opinion that swearing by divorce, when it is not intended to be a condition for divorce but rather a threat or an emphasis, has the same ruling as swearing by Allah. So, if the oath is broken, the one who swore must offer an expiation for an oath (kaffarat yameen), and the divorce does not take effect. When divorce is intended, they hold that divorce pronounced three times counts as one divorce.

As for the husband's statement, "I have sworn, and you will be divorced by three if this thing happens," if it is not a new conditional statement but rather an informing and reminding of a previous oath, then no divorce or prohibition results from this statement. In matters where there is a difference of opinion among scholars and the ruling depends on the husband's intention, it is advisable to consult trustworthy scholars in the questioner's country directly. The husband should avoid swearing by divorce and prohibition, because the lawful oath is swearing by Allah the Almighty.