certain在表示“确信、确定、有把握、一定会、无疑等”含义时，指确定无疑的，表示说话人有确切的理由或不容置疑的证据，语气较强，通常作表语，多用于肯定句或正式的书面语中，常可与positive 换用，但不可单独用作回答。有一下句型：
be certain that
be certain to do sth
be certain of doing sth
be certain of sth
be certain about sth
例如：他们确信会成功。
They are certain that they will succeed.
They are certain to succeed.
They are certain of success.
Are you absolutely certain about this?
你对这事绝对确信无疑吗？
It is certain that they will agree/They are certain to agree.
他们肯定会同意。
She's absolutely certain she's going to make it in the world...
她坚信自己会出人头地。
He is absolutely certain/positive that he has met her before.
他绝对肯定他以前见过她。
She's not certain/positive when the movie will begin.
她不确定电影何时开始。
It is certain/positive that every effect must have a cause.
毋庸置疑，有果必有因。
He is certain/positive that the fruit is ripe.
他确信水果熟了。
She looks certain to win an Oscar.
看来奥斯卡金像奖非她莫属。
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'm not certain who was there.
我无法确定谁当时在场。
sure指主观上对某事物的正确性或真实性没有疑虑的，常可与 certain 换用，但语气稍弱，通常作表语。
I am sure/certain that he is honest.
我肯定他是诚实的。
Are you sure/certain that this is the right road?
你肯定这条路对吗?
He wasn't sure/certain whether he would be able to get back as planned.
他不能肯定他是否能如期回来。
-Are you sure about that?
—-那事你能肯定吗？
-Positive.
—-能。
To make extra certain, a police helicopter kept watch from the skies.
为确保万无一失，一架警用直升机一直在空中警戒。
Victory looked certain.
胜利在望。
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