只能用certain的情形
以下情況通常只用certain,而不用sure：
1.當(dāng)句中用了形式主語或形式賓語 it 時.如：
It’s certain that he’ll come tomorrow.他明天肯定會來.
I thought it certain that he would be late.我肯定他會遲到.
It’s certain that prices will go up.價格肯定會上漲.
It’s not certain how much it will cost.這要花多少錢還不確定.
2.當(dāng)表示“某一”“某些”時.如：
A certain Mr Green wants to see you.有個叫格林先生的人想見你.
Certain plants are good to eat but others are not.有些植物好吃,而其他一些則不好吃.
三、只能用sure的情形
在 Be sure (not) to do sth.(一定要或不要做某事)這類祈使句中通常不用 certain.如：
Be sure not to forget it.千萬別忘記啦.
Be sure to get there before nine.務(wù)必在九點(diǎn)前到達(dá).
Be sure to turn off the light when leave.離開時一定要關(guān)燈.