核心问题
A Comprehensive Grammar of the English Language(p.1103)中说:“But that, like the preposition but, requires that the matrix clause precede it and be negative. (像介词 but 一样, but that 要求主句位于其前并且是否定句):
Nothing would satisfy the child but that I place her on my lap."
请问与 but that 连用的主句一定是否定句吗?
词典解答
“but that 要求其主句必定位于其前并且是否定句”的提法，指的是例外从句。其含义为“除了”。当 but that 作“要不是”解时，情形就不同了：
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But that he was prevented, he would have accomplished his design. (Mis) (主句后置并且是肯定句) 要不是他受到了阻止的话,他就完成他的设计了。
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He would have helped us but that he was short of money at the time. (ALD)(主句为肯定句)要不是那时候他没有钱,他就会帮助我们了。
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I'd have come with you but that I am so busy. (ALD) (主句为肯定句) 要不是我太忙，我就跟你去了。
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He would have said no but that he was afraid. (LDCE) (主句为肯定句) 要不是他因为害怕,他就说不同意了。