This "reverse engineering" logic is genius and unique to Jain’s pedagogy.
Learning a new language is like building a house. You can decorate it with fancy vocabulary later, but without a solid foundation—the grammar—the structure will crumble. For countless students of Hindi, that foundation is found in a single, seminal textbook:
The Blueprint of Fluency: Examining Usha Jain’s "Introduction to Hindi Grammar" Usha R. Jain's Introduction to Hindi Grammar