我一直搞不清楚 not to speak of, to say nothing of, not to say 之间的差别。
not to speak of = to say nothing of, 意思是“更不用说”“且不说”(not to mention)。既可用于肯定句之后，亦可用于否定句之后，这是跟 much more(用于肯定句之后)和 much less (用于否定句之后)所不同的。
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The scholar is well versed in Latin, not to speak of English which is his mother tongue. 那位学者精通拉丁语，至于他的母语——英语，那就更不用说了。
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He can dance, not to speak of singing. 他会跳舞，更不用说唱歌了。
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He has no scholarship, not to speak of experience. 他没有学问，更不用说经验了。
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It would take too long, to say nothing of the expenses involved. 这要费很长时间，且不说要花很多钱了。
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He hadn't seen the art galleries, the museums—to say nothing of the theatrical district. 他没有看到美术馆，也没有看到博物馆，更不用说剧院区了。
not to say 的意义则是完全不同的：“虽不能说”“即使不能说”：
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It is warm, not to say hot. 这天气虽说不上热，但是够暖和的了。
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He sounded impolite, not to say rude. 他那样说话，即使不算粗鲁，也是很不礼貌的。
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“The facts are singular, not to say grotesque,” said Holmes. 福尔摩斯说：“那些情节即使不能说荒唐，也是够离奇的了。”
(参见 to say nothing of, to speak of)
not until 见 not until, it was not until