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used to say that something can only happen or be true in a particular situation 除非;除非在…情况下 -
used to give the only situation in which something will not happen or be true 若非;如果不 Unless is used to talk about a situation that could happen, or something that could be true, in the future. If you know that something has not happened or that something is not true, use if… not If you weren't always in such a hurry (= but you are), your work would be much better. • Your work would be much better unless you were always in such a hurry.
Word Originlate Middle English: from on or in (assimilated through lack of stress to un-) + less.