Express Yourself Expressing certainty or uncertaintyExpressing certainty or uncertainty表示确定或不确定When you are stating what has happened or is going to happen, you can say how certain or uncertain you are about what you are saying:陈述已经或将要发生之事时对自己所言表示确定或不确定：
I'm sure/100% certain/absolutely positive I left my keys on the table.我肯定我把钥匙落在桌子上了。
There's no doubt in my mind that this is the best option.我心里确信无疑，这是最佳选择。
Without a doubt,/No question, this is where it was.毫无疑问，它原来就在这里。
I'm not (at all) sure what time they arrive, to be honest.老实说，我（一点也）拿不准他们什么时候到。
I rather doubt they'll call back.我有点儿怀疑他们是否会回电话。
(British English)
Extra Examples
He seemed strangely uncertain as to how to continue.很奇怪，他好像不知道该如何继续下去。
His research into the incident has left him uncertain as to exactly where responsibility lay.对该事件作了调查研究后，他无法确定究竟是谁的责任。
I feel deeply uncertain about the future.对于未来我深感信心不足。
We were rather uncertain of the direction it came from.我们都不太确定它是从哪个方向过来的。