Imbued with the vibrant Israeli spirit of the time, his classes became immensely popular. As German immigration also increased, Rosen taught more and more often. He became aware that teaching Hebrew was his vocation.

His love of the language made him a gifted teacher, whose classes were always in high demand. Deeply compassionate, he understood the difficulties faced by his students who had to learn a new and complex language as adults, far from home. He sought to make immigrants' first years in Israel pleasurable and rewarding - rather than frustrating - by helping them to reach language proficiency as quickly as possible.

Rosen worked tirelessly to hone teaching methods geared for adult learners. A highly successful multi-stage curriculum model was the fruit of his labor. It is still used by many modern institutions. In 1950, the Hebrew University in Jerusalem invited him to teach foreign students.

He became head of the Hebrew Department, a position he retained until his final day. The Ministry for Education and Culture commissioned his texts to answer the needs of rising numbers of immigrants arriving during the 1950s. In the 1980s, his Thousand Hebrew Words series of textbooks became bestsellers, and are still recognized as the definitive Hebrew tuition text for immigrants and foreign students.

His many writings include theoretical works translated into seven languages - and See, Hear and Read - for tuition of dyslexic students. He wrote numerous articles addressing specific problems in adult language instruction, and was the first teacher invited by Edwin Samuel (2nd Viscount Samuel) to broadcast radio classes during the British Mandate.

In Rosen Memorial Volume - Essays on the Teaching of Hebrew as an Additional Language (1975), Rosen is described by Ben-Zion Fischler and Uzzi Ornan:

"Aharon Rosen loved his country and his people, and saw his role in teaching new Israelis to speak Hebrew as his service to the nation. He proudly fought for Jerusalem in the War of Independence as a member of the Haganah.

"Rosen's passion inspired his students, his colleagues, and all those who came into contact with him on his many trips through Diaspora states. (...)

"... At the World Hebrew Foundation congress in Jerusalem, a representative who had traversed the Iron Curtain spoke, telling the participants how he personally had learnt Hebrew from Rosen’s radio broadcasts. He asked that Rosen stand, and thanked him on behalf of all Russian Jews for the invaluable gift he had given them. An emotional Rosen responded: "Your gratitude has made my life worthwhile."