Here's why:
* Subject-verb agreement: The verb "is" is singular, but "I" is singular. They don't agree.
* Sentence structure: "Is" usually needs a subject after it, and "I" can't be used as an object.
Here's how you could fix it, depending on the context:
* If you're using it as a subject: "I am..."
* If you're using it as part of a compound subject: "He and I are..."
* If you're using it in a sentence about something else: "It is I who..."
Example:
Incorrect: Is and I going to the store?
Correct: He and I are going to the store.