Complex sentence: Indeed, when the young Chaim applied to Yeshivas Chachmei Lublin, its Rosh Yeshiva of Daf Yomi fame, Rabbi Meir Shapiro who tested him, was astounded by his vast proficency in all aspects of Torah, knowing the entire Talmud by heart.
Split into simpler sentences: Indeed, when the young Chaim applied to Yeshivas Chachmei Lublin, he was tested by its Rosh Yeshiva of Daf Yomi fame, Rabbi Meir Shapiro. The latter was astounded by Chaim's his vast proficency in all aspects of Torah, knowing the entire Talmud by heart.