●问:有这样一道题： He told me that he wouldn't go to the party ____ invited. (A. unless was B. Until C. without D. if not)
我无法确定哪一个是正确答案，我觉的应选if not,而without显然是不对的。烦请解答。 ☆答：我也觉得选 if not是对的，without无论作介词或副词，在这里都不大可能出现；如用without ，后面应是 without 之类。
unless和if not在多数情况下是通用的，条件是用它引导一个与主句所述相反的陈述。
例如:Unless you take more care, you 'll have an accident.(=if you don't take more care.)
I want you to keep working unless I tell you to shop.(=if I don't tell you to shop.)
Unless you call me to say you're not coming, I'll see you at the cinema.(=if you don't call me to say you're not coming.)
但是在"虚拟语气"的条件句里，unless 不能代替if not 。
例如:
John would be our best play _if he weren't so lazy.(不能用unless 代替）
在if相当于 whether 时（在间接问句中），不能用unless代替if。例如：
She promised to let me know if she wasn't coming.（不能用 unless代替）
在心里存在否定时，不用unless 代替 if not。例如：
What shall we do if they don't answer our letter?(不能用unless )
至于until ，着重的是时间，不象 unless那样着重条件。如果此处用 until，我未曾见过 until后直接用过去分词的例子。用 unless instructed 似乎能成立。Longman Dictionary of Contemporary English(1987)有例句:Don't leave the building unless instructed to do so. 但是不能填unless was ，我想不起有 unless was invited这样的结构。
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