核心问题
（a）Nobody but me could nurse you. (Lewis)
(b) No one but can tell. (RWRT)
表示“例外”含义的 but 是个介词,为什么它的宾语竟用主格(I)呢?
词典解答
有人把作“除了”解的 but 看作是介词,而有的人则把它看作是连词。所以出现了上面用法上的差异。
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Everyone but me was tired. = Everyone was tired but me. (CGEL) 除我之外,每个人都累了。
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No one but me saw him. (Palmer) 除了我之外,没有人看到他。
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No one but he(him) showed much interest in the proposal. (OSD) 除他之外,没有人对这个提议表现出很大的兴趣。
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All but he had fled. (Fowler) 除他之外,所有的人都逃走了。
传统语法学家们主张在主语区 but 的后面用主格，在宾语区 but 的后面用宾格。
Nobody but she can solve our problem.
Nobody can solve our problems but her. (CGEL)
除她之外，没有人能解决我们的问题。