看来，曹老师对此耿耿于怀，哈哈。好了，我给您出处：

Until now, the BBC has only been available in South Africa on short wave.

直到目前为止，南非只能以短波形式收听英国广播公司的节目。

《柯林斯词典》

上句用于现在完成时，unitl now 等于 up to now 之意。表状态的谓语跟until 连用，表示该状态持续的时间。所以，之前改写的这个句子，也没有问题：

Commentators called the attack a second Pearl Harbor, and until now it has been our most tragic hour.

但是，第一句则不同：非延续的provide 加not，跟until 连用，not...until...表示“到现在已经提供出”，“此前已经提供出”。

【补充解答】以下是我个人对until 这个词的基本认识，特此请教曹老师，请批评指正：

▲ until 表示时间，既是介词，又是连词，意思是“到……为止；在……之前”，说明动作持续到某个时间为止。注意：“为止”的含义是：到此为止，不再继续。

关于until now， 它可以表示up to now 之意，用于现在完成时态，表示一个持续的动作至今“为止”，其后怎样不得而知。

现在完成时，本身就表示一个动作的结束。若是状态动词跟“时间段”连用，则表示持续到该时间为止，但无法判断该状态是否还在继续。例如：

Until now, I have studied English for five years. 现在是否还在学习英语，不清楚。

Until now I have always lived alone. 直到现在，我一直独自生活。

如果到现在还在继续中，则用“现在完成进行时”表示：

Until now, the Americans have been playing a very delicate political game.

直到现在，美国人一直玩着精心设计的政治把戏。《柯林斯词典》

▲ 有人总是把 not 和 until 放在一起说事，认为它们原本是一个固定的句型，事实并非如此。很多情况下，延续性动词跟 until 连用是不用 not 的，例如：　　

They worked until late into the night. 他们一直工作到深夜。　　

She lived here until 2010. 她住在这儿一直到2010年。　　

谓语动词具有延续概念时，可以用否定，也可以不用否定，但意思有差别。例如：　　

We waited for him until ten o’clock last night. 我们昨晚一直等他到10点。　　

We didn’t wait for him until ten o’clock last night. 我们昨晚没有等到他10点。

▲ 以下4个句子诠释了until 的基本用法。

They lived here until 1980.他们在这里一直住到1980年。（1980年之后就离开了。）

They didn’t live here until 1980.他们在这里没有住到1980年。（可能在1979年就搬走了。）

They didn’t begin to live here until 1980.他们直到1980年才开始住在这里。（1980年以前没在这里居住。）

They began to live here until 1980. （这是一个错误的句子，因为begin是一个非延续性动词，肯定状态下不可与until连用。）

【结论】延续性动词，如live, wait, work, study, stay, sleep，be 等跟until连用时，肯定、否定都可以。肯定句表示这个动作一直持续到这个时间，然后就停止了；否定句表示这个动作没有持续到这个时间，即在这个时间之前就停止了。

非延续性动词，如begin, finish, stop, leave, arrive, return, get off 等跟until连用时，这些动词只能用否定形式，不能用肯定形式。只有否定意义才表示这类动词持续的状态，说明这些动作直到这个时间才发生，之前没有发生。

▲ 一般情况下，把until当作before 来理解就可，不管动词是延续还是非延续，也不管句中有没有not。例如：　　

Please stay here until they come back. 在他们回来之前，请一直呆在这儿。

We waited until the ambulance arrived. 我们一直等到救护车来。

Don’t get off the bus until it stops. 在车停下来之前不要下车。（意味着：直到车停下才可下车。）　

It didn’t stop raining until the end of last month. 在上月底之前，雨从未停止。（意味着：直到上月底雨才停下来。）

下面是Cambridge International Dictionary of English 在 until 词条的解释，可供参考：

Common mistake: until now or so far?

Warning: choose the correct adverb!

Use “until now” to talk about a situation that existed, but has just ended:

The weather was lovely until now. I hope the rain stops soon.

To talk about a situation that still exists and may continue to exist, don't say “until now”, say so far:

W: The weather has been lovely until now. I hope it will continue.

R: The weather has been lovely so far. I hope it will continue.

如何准确理解下面句子的含义：

Until now, the above documents and information have not been provided by the Banks. 现在这些文件和资料提供了没有？

参考译文：在现在时间之前，以上文件和资料都没有提供。（意思，现在已经提供了）(until now 是句子谓语动词动作结束时间）

Until now, the BBC has only been available in South Africa on short wave.现在在南非，中波能收到BBC广播吗？

参考译文：在现在时间之前，在南非仅仅短波能收到BBC(意思，现在不是仅仅短波可以收到BBC)(until now 是句子谓语动词动作结束时间）

尤其是第二句。请资料齐全的老师提供可靠的权威解释。

until now到现在为止，迄今

Until now, a devaluation of the dollar seemed inconceivable.  

直到现在，美元的贬值似乎都无法想象。

It has taken until now to pin down its exact location.  

直到现在才确定了它的准确位置。

Until now television had fought shy of covering by-elections.  

迄今为止电视台一直竭力避免报道补缺选举。

They have threatened military action but held off until now.  

他们扬言要采取军事行动，但一直拖到现在。

Until now, the Americans have been playing a very delicate political game.  

直到现在，美国人一直玩着精心设计的政治把戏。

I didn't know it until now.  

今乃知之.

Until now, the government has only enforced the ban with regard to American ships.  

到目前为止，政府仅仅对美国船只实施了禁令。

Until now, the BBC has only been available in South Africa on short wave.  

直到目前为止，南非只能以短波形式收听英国广播公司的节目。

She found herself thinking thoughts that would never have entered her mind until now.  

她发现自己正在考虑一些以前从来不曾有的想法。

Farm life did not permit long fingernails. Until now it hadn't mattered.  

干农活不能养长指甲,至目前为止,她从来没有在乎.

Until now,crop breeders have paid little attention to inducible resistance.  

直到现在为止，作物培育者还很少注意诱导抗性。

Until now I've always credited you with more sense.  

在这以前,我一直都认为你不至如此糊涂.

Until now you've stayed away from the people you knew in prison. 

到现在为止,你一直避着在狱中认识的那些人.

And his elation, suppressed until now stimulated him.  

刚才还不敢太得意的心情, 一下子就扬扬得意了起来.

I was unaware of his existence until now.  

现在我才知道有他这麽个人.

(1) until   prep.直至；直到；在…之前

(英文解释: If something happens until a particular time, it happens during the period before that time and stops at that time)

A:介词用法 （until+名词/名词短语）(句子谓语动词为持续动词，动作一直持续到until 所指的时间结束。即until +时间 为句子谓语动词结束的时间）

Until 1982 he was senior lecturer in botany at Durham University. 

1982年以前他一直是达勒姆大学生物系的高级讲师。

consumers who have waited until after the Christmas holiday to do that holiday shopping. 

等到过完圣诞节才进行节日采购的顾客

B:连词用法（until+从句)(主句谓语动词为持续动词，动作一直持续到until 所指的时间结束。即until + 从句，为主句谓语动词结束的时间）

I waited until it got dark.

 我一直等到天黑。

Stir with a metal spoon until the sugar has dissolved. 

用金属调羹搅拌至糖融化。

(2)介词（与否定词连用）直到…之前，直到…为止，除非

(英文解释: You use until with a negative to emphasize the moment in time after which the rest of your statement becomes true, or the condition which would make it true)

A:介词用法（until+名词/名词短语）

The traffic laws don't take effect until the end of the year.

交通法要到年底才生效。

It was not until 1911 that the first of the vitamins was identified. 

直到1911年才发现了第一种维生素。

B:连词用法（until+从句)

The EU will not lift its sanctions until that country makes political changes. 

除非该国实施政治改革，否则欧盟将不会解除制裁。