If you are afraid of someone or afraid to do something, you are frightened because you think that something very unpleasant is going to happen to you.
She did not seem at all afraid.
她好像一点儿也不害怕。
I was afraid of the other boys.
我害怕其他的男孩子们。
If you are afraid for someone else, you are worried that something horrible is going to happen to them.
She's afraid for her family in Somalia.
她为她在索马里的家人担心。
If you are afraid that something unpleasant will happen, you are worried that it may happen and you want to avoid it.
I was afraid that nobody would believe me.
我担心没有人会相信我的话。
I'm afraid
If you want to apologize to someone or to disagree with them in a polite way, you can say 'I'm afraid'.
We don't have anything like that, I'm afraid.
我们恐怕没有任何像那样的东西。