既然有be sure of sth,为什么宾语从句要用be sure that而不是be sure of that呢?

1.按语法规定的确是be sure of that从句,但是在英语中如果介词后加的是宾语从句的话,一般规定必须去掉这个介词,所以正确的写法还是be sure that从句.
2.类似用法有很多,例如：
1）be afraid of...,be afraid that...
2）be surprised at...,be surprised that...
3）be aware of...,be aware that...