I✀m afraid that I can✀t.

I'm afraid 译为“我害怕”，而我害怕什么呢，所以“that I can't”做afraid 的宾语，所以为宾语从句

形容词后面的从句，听起来好像名词性质的短语的感觉。这种说法不正确，that起到连接作用，实际上后面是宾语从句相当于一个完整句子，意思就是说I'm afraid that I can't (do that)

有部分形容词是需要宾语的，形容词的宾语大多是动词不定式和从句，比如：
The problem is difficult to solve.
She isn't aware that she did wrong.
希望我能帮助你解疑释惑。

这是一个宾语从句，详解如下：

宾语从句的种类：
即该从句可以充当动词，介词或形容词的宾语
例： We heardthat he would come here lateron.
Tom told me (that) he would leave Changsha Airport at 8 pm.
She saidnothing aboutwho broke the glass.

I am afraid(that) he will fail in theexam.

这是我以前整理的材料，请放心采纳~

be afraid +that ( 宾语从句），记住就是了。
相似的用法还有
be sure +that
如：
I'm sure that he is from Canada.