I was not sure/certain what I ought to do.

问题描述:

I was not sure/certain what I ought to do.
为什么这样was not
可以是I am not sure /certain what I ought to do.

我不确定我该做什么.
was过去式 sure的否定式就是前面be加not啊.请问要的是这个解释么?ps:可以啊