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be ignorant of sth 是正确用法，有时可见be ignorant of the fact that...一般不用be ignorant that 从句
主语+be+adj+that 从句的用法
主语是形式主语可以，如 it ,语法叫分裂句，例句：
It is important that we should work hard.
如果主语不是形式主语，那么that 后一般是：
1）结果状语从句,相当于 so, that,：He was busy that he even didn't answer his girl friend's phone. 他很忙乃至没接女友电话。
2) 原因状语从句,相当于because that：He was confused that he was rejected for no reason. 他因无理由被拒绝而困惑。