【问】 A Comprehensive Grammar of the English Language(P. 1103)中说：“But that，like thepreposition but，requires that the matrix clause precede it and be negative：(像介词but-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?

【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：

1. 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。

2. 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。

3. I'd have come with you but I am so busy. (ALD)(主句为肯定句)

要不是我太忙，我就跟你去了。

4. 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。