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