I wrote आमर के पिता किताब लिख रहे हैं but it was not accepted. Why is एक necessary here?
I also think Duolingo has shown itself to be fickle on numerous occasions. I also wrote "आमर के पिता किताब लिख रहे हैं " and it WAS accepted.
But my question is this: why is"हैं" used here instead of "है"? Aamir, father and book are all singular.
It's the plural of respect for Aamir's father. That's why you write रहे and not रहा. Actually, रहे है gets corrected automatically by the Google keyboard because it's a grammatical error.
Bcos the sentence contains "a" book