If you want to be certain of getting a ticket, book now.

要想有把握买到票，现在就得预订。

I can be certain of success.

我一定能成功。

You can be certain of that.

你可以确信这一点。

But nobody can be certain of the outcome.

但是，谁也不确定这就是结局。

It's importance to be certain of this question.

它的重要性，不能确定这个问题。