It isn't quite_______that he will become well soon.

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.