如何区别not.until与not.unless的用法

until 的意思是 up to the time when（直到……时为止），表示时间。如：
(1) The hot weather lasted until September. 炎热的天气一直持续到9月。
(2) I waited until three o’clock, but he didn’t come. 我一直等到3点，但他没来。
(3) The air becomes thinner and thinner until it disappears. 空气越来越稀薄，直到消失。
not…until…的意思是：直到……才……；在……之前不要……。如：
(1) They didn’t find her until the next day. 他们直到第二天才找到她。
(2) She won’t go home until after the exam. 她要考完试之后再回家。
(3) I didn’t believe it till I saw it with my own eyes. 我原先不相信，直到亲眼看了才相信。
(4) I didn’t miss the key until I got home and found it wasn’t in my bag. 直到我回家发现钥匙不在口袋里才知道丢了。
unless 的意思是 if…not（如果不……），表示条件。如：
(1) You will fail unless you work hard. 你若不努力就会失败。
(2) Come tomorrow unless I phone. 如果我不打电话，你明天就来吧。
(3) I’ll take the job unless the pay is too low. 如果工资不是太低，我会接受这一工作的。
not…unless…的意思是：除非……否则不……。如：
(1) Don’t act unless you’re certain. 没有把握就不要做。
(2) I won’t go unless I’m invited. 除非请我，不然我就不去。
(3) She will not work hard unless you push her. 除非有人督促她，不然她不会努力。
(4) The targets would not be achieved unless other departments showed equal commitment. 除非其他部门同样努力，否则这些目标无法达成。
(5) He won’t want to eat it unless he’s really hungry, in which case he’ll eat almost anything. 除非很饿，否则他不会吃东西，一旦饿了，他几乎什么都吃。
但在有些情况下，用 not…until…和 not…unless…都可以，只是说话者的侧重点稍有不同。如：
(1) Don’t start work again until you’re completely better. 在病没完全好之前，请不要开始干活。（侧重时间：开始干活的时间是病好之后）
(2) Don’t start work again unless you’re completely better. 除非病完全好了，否则就不要开始干活。（侧重条件：开始干活的条件是病好）
典型考题：
下面两道题答案都只能选 until：
(1) I didn't manage to do it ______ you had explained how.(1987年高考题)
A. unless 　B. until 　C. when D. since
(2) —Was his father very strict with him when he was at school?
—Yes. He had never praised him _______ he became one of the top students in his grade. (2003北京春季高考题)
A. after B. unless C. until D. when
这两题要选 unless
除了与句意有关外，还与句子时态有关。在这两个句子中，如果要填 until，就说明说话者强调条件，如果是这样的话（联想大家都熟悉的“主将从现”），主句的时态就可能考虑要用将来时态（比如过去将来时、过去将来完成时等）。比如：
(1) I wouldn’t have managed to do it unless you had explained how.（此句只是为了顺应语意而写，其实这句是不对的，因为这样一来，句子就成了虚拟语气，而 unless 通常不用于虚拟语气，正确的写法是：I wouldn’t have managed to do it if you hadn’t explained how.）
(2) He would never praise him unless he became one of the top students in his grade.（这句的主句谓语用 would never praise，后面用 unless 来强调条件就对了）