答案：
壹、 先弄明白它們意思與詞性：
sure
adj.對....有把握, 確信某事, 穩當的, 可靠的
adv.的確, 當然
certain
adj.確定的, 某壹個, 無疑的, 必然的, 可靠的
pron.某幾個, 某些
二、 用法：
（壹）Sure與certain是近義詞，均作“壹定的，確信的”等解。兩者用法相似，多數情況下可換用，但有時不能換用。其用法區別大致如下： 壹、 兩者可換用的情況：
1． 跟不定式，可用人或物作主語。例如： It is sure/certain to rain. He is sure/certain to come.
You’re sure/certain to be happy with them.
2．跟連接代/副詞引起的不定式，常用人作主語。例如： I’m not sure/certain what to do.
He was not sure/certain when to start.
3．跟of或about引起的短語。sure後可跟of doing sth. 常用人作主語。例如： I’m not sure/certain of that. Are you sure/certain about it? I’m sure of his coming. He’s sure of winning.
註：sure to do sth.與sure of doing sth.的區別：前者表示說話者認為“主語壹定…”；後者表示“主語自認為壹定…”。例如：
He is sure to succeed. (說話者認為“他”壹定會成功。 He is sure of succeeding. (“他”自己認為壹定成功) 4．跟that引起的從句，常用人作主語。例如： I’m sure/certain that he’ll come.
I was sure/certain (that) he had seen me.
I’m quite sure/certain I left the book on the table.
註：當sure或certain用於否定句時，其後跟whether或連接代/副詞引起的從句，不跟th
that引起的從句。例如：
I’m not sure/certain whether I’ve met him before.
I’m not sure/certain whether he will come.
I’m not sure/certain who he is.
I was not sure/certain what I ought to do.
I’m not sure/certain why he wants it.
以上幾種情況中的sure與certain雖可換用，
但隱含意思不同。
Sure
表示主觀的推測或感覺，
所述事實並不壹定如此；
certain
則指說話者有充分的理由或根據，肯定某事會發生。如下面兩句中的certain均不可用sure替換。例如：
I’m certain he wasn’t in the classroom. I saw him in the library.
I’m certain than he will go the co
ncert. He told me so an hour ago.
5．與make連用構成短語，後跟從句，作“弄清楚；設法做到”解。例如：
Make sure/certain when the train leaves.
Have you made sure/certain that the train has already left?
6．與for連用構成短語，在句中作狀語。作“確實；毫無疑問”解。例如：
I don’t know for sure/certain that he’s dead.
I cannot sau for sure/certain when he will come.
（二）、兩者不可換用的情況：
1在“It is…that結構中只能用certain，不能用sure。
例如：
It is certain that he will come.
It was certain that our football team would win the game.
如是否定句，用whether，不同that。例如：
It is not certain whether he will come.
2． 在祈使句中通常用sure，不用certain。例如：
Be sure to tell me all the news.
Be sure and come as soon as possible.
Be sure you don’t upset any of them.
3．作定語時，sure作“有把握的；肯定的；可靠的”解；certain作“某個；某些”
解。例如：
Put it in a sure place.
There was only one sure way to succeed.
A certain person called on me yesterday.
Certain students have failed in the exam.
4．Sure在美語中可用作副詞作肯定答語，用以代替surely，相當於certainly，of course
；而certain則不能用作副詞。例如：
I wonder if I could use your phone.
Sure. Go ahead.
Zhou Lan, can I have a look at your copy of China Daily?
Sure, go ahead.