If "certain" means "sure" or "confident":

* Uncertain

* Doubtful

* Unsure

* Indecisive

* Hesitant

* Ambiguous

* Vague

If "certain" means "definite" or "specific":

* Uncertain

* Indefinite

* Unclear

* Ambiguous

* Vague

* General

* Non-specific

If "certain" means "inevitable" or "guaranteed":

* Uncertain

* Doubtful

* Possible

* Contingent

* Unlikely

* Improbable

So, the most appropriate opposite will depend on the specific situation and the meaning of "certain" in that context.