I assume this will come later in the course but such a verbal form/tense exists in Hindi indeed. I think the following would work (for a male speaker here): मैं खाता हूं (I eat) vs मैं खा रहा हूं (I am eating)
I answered "I eat banana". How does केला have a definite article here?
This sentence can mean both “I eat the banana” and “I eat a banana.” However, you can’t write “I eat banana” because that’s just not proper English :)
'I eat bananas' is though; yet it's incorrect. Is it correct really, or would it be different if plural?
Yes. The Hindi sentence would have to be different for you to be eating bananas rather than a banana.
Hindi doesn't have an explicit definite article so you have to put one in there when translating as appropriate.