What is the ruling on a husband swearing to make his wife unlawful to him if she does not wear the curtain (i.e., the hijab or veil), with the wife claiming illness and inability to wear it?

Prohibiting one's wife depends on the husband's intention. If he intended divorce, then it is a divorce. If he intended ẓihār (a pre-Islamic form of divorce by comparing one's wife to an unmarriageable relative), then it is ẓihār. If he did not intend either, then it is an oath. Divorce cannot be temporary, whereas ẓihār can be temporary, according to the more preponderant opinion, and its expiation becomes obligatory upon engaging in intercourse before the period expires. One must be careful not to act recklessly with words of prohibition or divorce to resolve marital disputes.