Here are some synonyms for "certain," depending on the context:

For sure, definitely, undoubtedly, assuredly, positively, unquestionably, undeniably, incontrovertibly, irrefutably, absolutely, completely, totally, fully, firmly, conclusively, decisively, incontestably

For "a particular" or "specific":

* Particular, specific, definite, precise, exact, distinct, individual, given, designated

For "sure" or "confident":

* Confident, sure, assured, convinced, positive, certain, satisfied, firm, resolved, unwavering

The best synonym will depend on the specific context and desired nuance.