...it would be unforgivable to miss it (= the train). (New Concept English, Book 3, L. 60) 这里为什么要用虚拟语气 would be?
充当主语的动词不定式有时表示真实的情况，有时则表示假设的或可能发生的情况。在后两种场合下，谓语动词要采用虚拟语气。例如：
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The molecules of water are incredibly minute. It would take about 125 million of them to form a row an inch long. 水分子小得难以置信。把水分子排成一英寸长就需要大约一亿二千五百万个分子。(动词不定式短语所表达的事情是假想的,不是真的要去做的)
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...it would perhaps be unwise to forecast undue restrictions on the nature of the ultimate achievement. ……现在便对其最终成果的性质预先定下不适当的框框，也许是不明智的。
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It would be impossible to estimate how many crimes went undetected last year. 要想估计出去年有多少犯罪活动未被发现,那是不可能的。
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It would be a shame to stay indoors on such a beautiful day. (Chambers) 这样美好的时光, 要是呆在家里, 那就太可惜了。
请做下面的英语试题：
- "Tell the nurse you've been waiting an hour."
"Not now. She's talking to a patient, and ____.
A. it would be rude interruption B. interruption would be rude of me
C. if I interrupt would be rude D. it would be rude to interrupt(TOEFL)
- It would be ____ a risk to let the child go to school by himself.
A. following B. passing C. running D. carrying(Nelson, 500B)
(参见 subjunctive mood B 项)