Explain what is wrong with the following sentence and make any corrections necessary: "I wanted to stay for dessert, but I taking an exam early the next morning."

You're missing the helping verb "am" or "was."

. . . I am taking . . .
or
. . . I was taking . . .

"I want to stay for dessert, but I have to take an exam tomorrow morning"

The following sentence has a subject-verb agreement error. The verb form "taking" does not match the subject "I." To correct it, you need to use the correct verb form that agrees with the subject.

Corrected sentence: "I wanted to stay for dessert, but I was taking an exam early the next morning."