* His mother's influence: While not a teacher in the academic sense, his mother fostered his intellectual curiosity and supported his pursuits, even when others discouraged him.
* K. Sankara Iyer: A local mathematician who gave Ramanujan some grounding in more advanced concepts.
* S. L. Loney's books: Ramanujan learned much independently from Loney's trigonometry and analytic geometry textbooks. These books served as his primary instructors in a way.
* G. S. Carr's "A Synopsis of Elementary Results in Pure and Applied Mathematics": This book was incredibly important. It contained a vast collection of theorems and formulas, without many proofs. Ramanujan worked through this book intensely, discovering and developing his own proofs and methods. It served as a launchpad for his independent mathematical explorations, making Carr arguably the most impactful "teacher" in terms of content learned.
* Professor G. H. Hardy: This is arguably the most crucial influence. While not a teacher in the traditional sense, Hardy became Ramanujan's mentor and collaborator at Cambridge University. Hardy provided Ramanujan with formal mathematical training and guided his work within the established mathematical community.
In short, Ramanujan's education was largely self-directed, with significant influences from books, a few local mathematicians, and ultimately, the guidance of G.H. Hardy. There wasn't a single person who "taught" him in the way a student typically learns from a professor.