I am afraid we have to go now的afraid后面为什么不加to或of

I am afraid 后省略了of , (that) we have to go now 是介词of的宾语从句
这是一种固定用法。一些be +形容词 +介词的短语，常常省略介词再接that引导的宾语从句。如
be sure of/ about sth, be surprised at sth, be pleased with 等等
I'm sure (that) you are honest
I'm surprised （that) his father is an American
I'm pleased ( that) he finished the task in time.
如果这类短语后接特殊疑问词引导的宾语从句，则不省略介词
I'm pleased with what you said

be afraid of sth 害怕某物,某事
be afraid of doing sth 做了某事感到害怕
be afraid to do sth 由于害怕而不敢做某事
be afraid that 从句, 这句话是be afraid后面接的省略that的宾语从句

希望能帮到你，祝更上一层楼O(∩_∩)O
有不会的欢迎再继续问我(*^__^*)

I am afraid + 句子“ 恐怕”

起着缓和语气的作用。如果你有对方不太爱听的事要告诉对方的话，最好用这个结构。 如：

I am afraid I can't lend it to you.

你说的哪两种结构没有此功能

请及时采纳，不懂继续问( 天天在线 ）(*^__^*) 祝学习进步! 谢谢！

afraid后的是一个从句，可以理解为afraid后省略了连接词that

be afraid +从句
这句话可以写成 I'm afraid to go now