"आमिर के पिता एक किताब लिख रहे हैं।"

Translation: Aamir's father is writing a book.

[Udbav1]

Why is this under comparison?

[Myusernametoday]

Why is it in plural

[Myusernametoday]

Is it for respect to Aamirs dad in that sentence

[DevendraPaliwal]

Yes.

[LaviT333]

I don't see the plurality in this sentence

[DevendraPaliwal]

"लिख रहें हैं" is plural form. Singular form will be "लिख रहा है"

[ArunavaC]

Precisely. Third person usage, is the person is respectable/elderly, usage changes accordingly.