But I'm sure you (NOT/HEAR) of Cornish.

Given the text provided, it seems like a sentence fragment is presented with a gap that needs to be filled using the correct form of the verb in the brackets. The complete sentence should read: "But I'm sure you **haven't heard** of Cornish." Explanation and Solution: - The structure of the sentence indicates it is in the present perfect tense. - The negative form is required here because it says "But I'm sure," which implies a negation or denial of the action. - The verb "hear" should be conjugated in the present perfect form since the action of not hearing about something has relevance to the present moment. Therefore, the correct form to fill in the gap is "haven't heard." The sentence would be correct when completed as: "But I'm sure you **haven't heard** of Cornish."