Question: Am I smarter than you? (Mentally, put the words in regular order: "I AM smarter than you." You can see that the verb is "to be." So use "to be" in the answer. Do NOT use do / does / did when the verb is "to be.")
Is Tom taller than you? Yes, he IS.
Are we younger than James? Yes, we ARE.
Was yesterday sunny? Yes, it WAS.
Am I handsome? No, you ARE not.
Will tomorrow be Friday? Yes, it WILL / WILL BE.