Does a divorce occur if a man says to his wife's father, "If what you say is true, then my wife is divorced," then after a long time he forgets the details of the conversation and whether what he said was a lie or the truth, knowing that the wife was not present?

If you made a divorce conditional on the truthfulness of your wife's father, then doubted your own truthfulness and forgot the matter upon which the divorce was conditioned, the divorce does not occur. This is because the breach of the oath is uncertain, and the default is the continuation of the marital bond. Your wife remains under your responsibility. We advise you to avoid swearing by divorce in the future and to be wary of obsessive doubts.