Does divorce take effect if a man says to his wife: "I swear by divorce that if you do such and such, you will be divorced and forbidden to me according to all schools of thought," and the wife then does that thing?

Conditional divorce, according to the majority of scholars, occurs if the condition it is tied to is met, and this is the prevailing fatwa. So, if a husband says to his wife, "You are divorced if you do such-and-un-such," and she does what was sworn upon, the divorce takes place.

As for his statement, "And you are unlawful for me," the preponderant view is that it depends on the husband's intention: 1. If he intended ẓihār (a pre-Islamic form of divorce by likening one's wife to one's mother), then he must pay the expiation for ẓihār. 2. If he intended divorce: if he intended it as an affirmation of the first divorce, then only the first divorce occurs; if he intended to initiate another divorce, then two divorces become binding upon him. 3. If he intended an oath by Allah Almighty or did not intend anything specific, then he must pay the expiation for breaking an oath.

His statement, "according to all schools of thought," is an emphasis of the oath, and nothing additional results from it.

In the case of a divorce that is sworn upon, there is no breaking of an oath in the sense of expiation. If the wife does what was sworn upon, the divorce occurs. If he says, "I divorce you thrice if you speak to Zayd, you are divorced," then if she speaks to Zayd, one divorce becomes binding upon him unless he intended more.