I am afraid that 后面加从句
例如 I am afraid that you are right.
我恐怕你是对的.
I am afraid not.是一个完整的句子.
例如：-Do you think it is going to rain?
-I am afraid not.
你认为天将要下雨吗?
我恐怕不是（这样）的.（即否定）