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”，有时可互换。互换的场合应注意以下几点用法：

1. 表示说话者的态度或看法

即表示说话者自己的态度或看法，其意为“一定会”“肯定会”。此时主要用法有：

(1) 单独用作表语。如：

One thing was sure [certain]: they'd be late. 有一件事是确定无疑的，他们会迟到。

(2) 后接不定式。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。

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。

(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?

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。

(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。

二、只能用certain的情形

以下情况通常只用certain，而不用sure：

1. 当句中用了形式主语或形式宾语 it 时。如：

It's certain that he'll come tomorrow. 他明天肯定会来。

I thought it certain that he would be late. 我肯定他会迟到。

It's certain that prices will go up. 价格肯定会上涨。

It's not certain how much it will cost. 这要花多少钱还不确定。

2. 当表示“某一”“某些”时。如：

A certain Mr Green wants to see you. 有个叫格林先生的人想见你。

Certain plants are good to eat but others are not. 有些植物好吃, 而其他一些则不好吃。

三、只能用sure的情形

在 Be sure (not) to do sth.(一定要或不要做某事)这类祈使句中通常不用 certain。如：

Be sure not to forget it. 千万别忘记啦。

Be sure to get there before nine. 务必在九点前到达。

Be sure to turn off the light when leave. 离开时一定要关灯。

四、两者在习语中的用法

1. 用于 for certain / for sure, 意为“肯定地”“确切地”等，两者可互换。如：

I can't say for certain [sure] when he will come. 我不敢肯定地说他什么时候来。

No one knows for sure [certain] what happened to her. 没有人确切地知道她出了什么事。

2. 用于 make certain / make sure，意为“弄清楚”“弄得有把握”，两者可互换。如：

They made certain [sure] (that) they weren't late. 他们有把握不迟到。

I think there's a train at 10:40, but you'd better make certain [sure] of it. 我想10:40有趟火车，但你最好去核实一下。