Most people who knew my grandfather, Rav Moshe Feinstein, zt"l, never saw him learn from regular seforim. He was always writing. All his learning took place in the past, before anyone who knew him in New York came on the scene. So, it was an enigma: When did he learn? On Shabbos, he did his Daf Yomi for the entire continued on week. He learned the parsha with Onkelos and Rashi, and Midrash Rabba for that parsha. Every morning, he learned two perokim from Tanach, just the words. He had, in fact, an encyclopedic knowledge of Tanach. He did no other learning, certainly not Kabbalah, the mystical, esoteric teachings. I once asked him if he had ever learned Kabbalah, and he gave me a cryptic response: "Those who talk about Kabbalah don't know it, and those that know it, don't talk about it." He never answered the question directly, but an incident that took place in 1975 can give us a clue.
He had been asked by Avram Levine, who was then serving as secretary for my grandfather's organization, Agudas Rabbanim, to be mesader kedushin at his wedding. Avram Levine, who subsequently moved to Israel, where he writes for the Jewish Press, is a Lubavitcher chassid, and the wedding was to take place at Lubavitch headquarters in Crown Heights, Brooklyn. Arrangements were made so that, after the chupah, my grandfather, for the first time, would meet with the Lubavitcher Rebbe, Rabbi Menachem Mendel Schneerson, zt"l, who, as per Chabad custom, would not be personally present at the wedding. After the chupah, my grandfather and the Rebbe spent one hour alone together. Afterwards, my grandfather told me that the first few minutes were spent on pleasantries, each asking about each other's health and families. Then they discussed questions relating to Klal Yisrael, including recent political decisions and other matters of concern. Finally, the Rebbe told my grandfather he had a question relevant to Torah. He told my grandfather that, for 50 years, he had been perplexed by a major enigma in the Zohar, the foundational work of Kabbalah. His question dealt with the Kabbalistic concept of the Yanuka, a wonder child who possesses supernatural wisdom. My grandfather did not pause for a second before giving the Rebbe an extraordinarily brilliant explanation to the question that had been posed. The Rebbe was reportedly ecstatic with the response. Their meeting ended, and my grandfather left. Later, after he told me what had happened during their meeting, I asked my grandfather if he had learned this explanation at an earlier time, but my grandfather refused to discuss the matter further. When I discussed this incident with my uncles, my grandfather's sons, we concluded that it would have been impossible for my grandfather to have responded to the Rebbe's question so quickly and with such confidence unless it was already clear in his mind from earlier study. This means he must have studied Kabbalah extensively in his earlier years.
My grandfather, of course, spent just as much time answering questions from Jews who did not have the stature of the Lubavitcher Rebbe. On one occasion, he heard from a young man who lived in the Midwest who had, nebach, just lost his father. To complicate matters, the funeral was one week before the young man was scheduled to be married. The Shabbos of his aufruf fell in middle of shiva. Because his family was close with Rav Moshe, they placed a long-distance call to New York to discuss some specific shailos regarding the groom's circumstance. The young man was very appreciative of Rav Moshe's assistance and his warm words of chizuk. A few minutes after they had hung up, the phone rang in the young man's home. A long-distance operator asked the surprised young man if he would accept a long-distance call from Moses Feinstein. Rav Moshe explained his reason for calling. "After we hung up," he said, "I realized that this Shabbos is also your kallah's Shabbos Kallah. The custom is that the choson sends the kallah special flowers for this special Shabbos. If your kallah would not get the flowers, and all her friends who come for the Shabbos Kallah would see that she does not have them, she would feel bad. I wanted to tell you that you must make sure to send the flowers, despite your situation." Rav Moshe then went on to say that the halachos of aveilus (laws of mourning) are d'rabbanan (rabbinic), but making someone feel bad is an issur d'Oriesah (forbidden in the Torah).