核心问题
I was enough fool to believe her. (我真是太傻了,竟然相信了她。)形容词 enough 可以放在名词前,也可以放在名词之后,为什么说上面的句子有错误呢?
词典解答
当名词为不可数名词时，充当定语的 enough 既可放在名词之前，也可放在名词之后：
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We have enough money (money enough) for John to buy a car. (LDCE) 我们的钱足够约翰买辆汽车。
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There's enough food (food enough) for everybody. (ALD) 有足够的食物供大家吃。
当可数名词为复数时，enough 也是可以采取两个位置的(名词前或名词后)：
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We have enough seats for everyone. (LDCE) 我们的座位够所有人坐的。
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Have you had peanuts enough? 你有足够的花生吗?
但是,在你所提问的句子中,fool 只是貌似名词,实际上已具有形容词的特点,故 enough 只能放在它的后面:
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I was fool enough to believe her. 我真是太傻了，竟然相信了她。
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He wasn't man enough to admit his mistake. 他没有承认错误的大丈夫气概。
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He was fool enough to go out without a coat. (CGEL) 他真傻,竟然不穿大衣就外出了。
因此，说 enough fool，如同说 enough pen 一样，是错误的。