核心问题
（A）I don't think he has come.
(B) I think he has not come.
我认为动词 think 是可否定转移的动词, 因此, 应把否定词 not 放在主句中 (I don't think)。据我看, 句 (B) 是错句。
词典解答
句(B)也是正确的。但用得较少,句(B)的否定意义大大强于句(A)。因此,当需要强调从句的否定意义时,就可采用这种句式:
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I suppose you don't ever write to him. 我想你从来也不给他写信。
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I believe I haven't met you before. (CGEL) 我认为我从前没有见过你。
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She imagined we wouldn't say anything. (CGEL) 她设想我们不会说什么的。
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I think it isn't a good idea. 我认为那不是一个好主意。
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I thought he was not happy. (GCE) 我认为他并不幸福。
transferred negation