I'm not sure about that.
我想不大可行吧？
I wouldn't say that.
我不赞同。
I'm not sure that's the case.
我不确定是否是那样。
Why not?
为什么不？
It may not be that simple.
问题可能没那么简单。
I don't often use them.
我很少使用。
I wouldn't really know.
我不知道。
I'm not really sure.
我不大确定。
I wouldn't know anything about that.
我不知道。
There's nothing wrong with it.
那个没有什么不好。