天才们,看看这句英语错了没

应该用 I'm not sure what I should buy for my friend.

差不多。
修改一下应该是：I'm not sure that what to buy for my friend.
另一种说法： I wonder what to buy for my friend.
第二种地道一点。

I am not sure that what I should buy for my friend

这个句子应该是对的．of是介词，与what构成介宾结构

YES