关于这两个词是众说纷纭，甚至于有权威考试题目都被带进了沟里。先说一下相同点用法，基本可以互换：
一、两者在用法上的相同点
两者都可用作表语，表示“一定”或“确信”只是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。：
1. 后接不定式表示说话者的观点，而不是主语的观点。表示说话者的态度或看法，其意为“一定会”“肯定会”。此时主要用法有：
如：
He's certain [sure] to win. 他一定会成功。
They're certain [sure] to need help. 他们肯定需要帮助。
If you do this, you are certain [sure] to be found out. 如果你这样做一定会被发现。
Short of a miracle, we're certain[sure] to lose.
除非发生奇迹，否则我们输定了。
She's sure[sure] to be picked for the team.
她肯定会入选该队。
要注意主语是第一人称I时，是不是大家的所有人的观点或看法：
I’m sure to win.都认为我会取胜。
I’m sure of winning.我确信我会取胜。
2. 表示句子主语的判断或信念
即表示句子主语对某一情况的判断或信念，其意为“相信”“确信”“有把握”等。此时通常用于以下结构：
(1) 后接of [about] sth。如：
He is certain [sure] of success. 他确信会成功。
Are you certain of [about] that? 你对此有把握吗?
(2) 后接 of doing sth。如：
Our team is certain [sure] of winning. 我们队有把握赢。
You can be sure [certain] of his agreeing. 你可以放心他会同意。
比较同义句：
He is certain [sure] of winning.
=He is sure [certain] that he will win. 他自信会赢。
I was certain that love was quite enough to conquer our differences
我相信爱足以克服我们的种种差异。
注意：be sure about后不能接doing。这是它们的一个区别。
(3) 后接 that / whether / if 从句。如：
I am sure [certain] that he is honest. 我肯定他是诚实的。
I'm sure [certain] that it's not your fault. 我敢肯定这不是你的错。
Are you certain [sure] that this is the right road? 你肯定这条路对吗?
She's absolutely certain [sure] she's going to make it in the world
她坚信自己会出人头地。
I'm sure that you will get your wish.
我相信你会心想事成。
注：当be sure [certain]为肯定式或疑问式时，后接that从句;当be sure [certain]为否定式时，后接whether [if]从句。如：
I'm not sure [certain] whether he still works there. 我不能肯定他是否还在那里工作。
I wasn't sure [certain] whether he would agree. 我不太肯定他是否会同意。
He wasn't sure [certain] whether he would be able to get back in time. 他不能肯定他是否能准时回来。
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。
但是，下列句子都是摘自权威词典：
I'm not sure that this is the most valuable use of my time.
我不能肯定我的时间这样来安排最有价值。
I am not sure that this is sound democratic practice
我不能肯定这是正确的民主行为。
Confidentially, I am not sure that it wasn't above their heads.
私底下说，我不敢肯定他们明白了。
(4) 后接其他从句。如：
I'm not sure [certain] where she lives. 我不能肯定她住在哪里。
I'm not certain [sure] who wrote it. 我不太清楚这是谁写的。
3.make (it) certain [sure] that从句，确信，确保
Firstly, they must make certain that their pension needs are adequately catered for
首先，他们得确保他们能获得足够的养老金。
Certain that he was going to make good his threat to kill her, she lunged for the gun
他曾威胁要杀了她，而她确定他这次就要向她索命，便冲上前去拿枪。
Make sure that sign's the right way up.
一定不要把这招牌挂颠倒了。
Make sure that you bed the roots firmly in the soil.
一定要使根部牢牢地扎在土壤里。
Make sure that the firm's insurance cover is adequate.
确保该公司上了足够的保险。
Make sure that you follow the instructions carefully
一定要认真按照操作指南来做。
Make sure that you drink plenty of fluids.
务必保证足够的水分摄入。
Make sure that all gas fires and central heating boilers are serviced annually.
确保所有的煤气取暖器和中央供暖的锅炉每年都检修一次。
They made certain [sure] (that) they weren't late. 他们有把握不迟到。
These devices make it certain [sure]that the electricity in the wires do not start fire.
这些装置确保电线里的电不会燃烧。
I’m leaving.
Make sure[certain] you lock the door.
我要走了。那确保把门锁好。
Make sure you get a good look at their faces.
你一定要仔细看清他们的面孔。
Make sure you park your car clear of the entrance.
切莫把车停在入口处。
4.It is (not) certain [sure] that 从句，表示：某人确信，肯定……
It isn’t quite certain [sure] that he will be present at the meeting.
他不是十分确定能出席会议。
It is certain/ positive that every effect must have a cause.
毋庸置疑，有果必有因。
5.Be certain [sure] to,肯定也好，否定也好，都可以用在祈使句里：
Be sure to give your family my regards.
务必代我向你的家人问好。
Be sure to wear the correct footwear to prevent injuries to your feet.
一定要穿合适的鞋，以免脚受伤。
Be sure to store them carefully as they bruise easily.
小心存放，因为它们很容易被碰伤。
Be sure to make copies of all the papers.
一定要把所有文件都复印好副本。
Be sure to come. Don't let me down.
你一定来，可别拆我的台。
Be sure to do the sowing in good time.
确保适时播种。
Be sure to make him believe it.
务使他相信此事。
Be certain to eat slowly and chew your food extremely well
一定要慢慢吃，要细细咀嚼。
《柯林斯高阶英汉双解学习词典》
6.for certain(sure确切、确实
Hill had to find out for certain.
希尔得弄个清楚。
One thing is for sure ─ it's not going to be easy.
有一点可以肯定——事情不会很容易。
One thing's for sure, Astbury's vocal style hasn't changed much over the years
有一点可以肯定，阿斯特伯里的演唱风格多年来没怎么改变。
The museum's P.R. man was going to be dragged over the coals for sure.
博物馆的公关人员肯定会受到斥责。
如在否定句在，for certain(sure也在否定之内，“不确切，不肯定，不确实、说不准”如：
I can't say for certain when we'll arrive.
我说不准我们什么时候能到。
She couldn't know what time he'd go, or even for certain that he'd go at all
她不知道他何时走，就连他到底走不走都说不准。
No one knows for certain how many refugees wander the world today, or languish in camps without a permanent place of settlement.
无人清楚如今有多少难民在到处流浪，或有多少人流离失所，在难民营里受苦。
I couldn't say for certain when he'll arrive.
我说不准他什么时候到。
No one knows for sure what happened.
谁也不知道究竟发生了什么事。
I think he'll be back on Monday, but I can't say for sure.
我想他星期一会回来，不过我不敢肯定。
Even to this day we don't know what happened for sure.
即使到现在我们也不确定究竟发生了什么。
No one knows for sure why adolescence is unique to humans
没有人确切地知道为什么只有人类才会经历青春期。
We cannot say for sure what will happen
我们无法确定会发生什么事。
You can't say for sure that it won't rain tomorrow.
明天难保不下雨。
二、重要区别
1.作定语时,sure表示"可靠的,无疑的".而certain修饰可数名词时,意为"某一,某些",修饰抽象名词时,意为"有点,有些".如:
The student made a sure answer.学生做出了确凿的回答.
A certain Brown is waiting for you at the gate.门口有个叫布朗的在等着你.
It was a certain Dr Davis who performed the operation.
是一位叫戴维斯的医生做的手术。
It had a certain funky charm, I guess, but it wasn't much of a place to raise a kid.
我想那个地方是有种怪怪的独特魅力，但是不适合抚养孩子。
There were certain places that were really creepy at night