When expressing certainty, the phrase "am sure" can be replaced with several other synonyms. For instance, "I am positive" or "I am convinced" indicate that the speaker has no doubt about something. "I am certain," "I am confident," or "I am sure-fire" convey a strong sense of assurance. Another possible synonym is "I am confident," which denotes belief in one's own abilities or the expected outcome of a situation. "I'm satisfied" or "I'm convinced" can also express the same degree of certainty. Overall, there are various synonyms that can be used to replace "am sure," and the choice often depends on context and personal preference.