Does an oath take effect if a man says to his wife: "If you have done such-and-such and did not tell me, you are unlawful to me for the rest of your life," with the intention of threatening, not divorcing, knowing that she did indeed do that? And how can expiation be made for this statement?

Prohibiting one's wife depends on the husband's intention. If he intended divorce, it is a divorce. If he intended ẓihār, it is ẓihār. If he intended an oath, or did not intend anything, then an expiation for an oath (kaffārat yamīn) is required.

If the prohibition of the wife was conditioned on something that has already occurred, and the husband intended to threaten or intimidate without intending divorce or ẓihār, then an expiation for an oath is required of him.

The phrase "forbidden for life" is an emphasis on the prohibition and does not entail a separate ruling beyond the expiation for an oath.

It must be noted that an angry person is not held accountable for what he utters if he is unaware of what he is saying.