0 except if -- 除非；除了在…的情况下
Unless you call me to say you're not coming, I'll see you at the theatre (= I will see you there if you do not call to say you are not coming). 除非你打电话说你不来了，否则我就在剧院等你。
You can't get a job unless you have experience (= you can only get a job if you have experience). 除非有工作经验，否则你找不到工作。
