Hyman Kaplan, the irrepressible student at the American Night Preparatory School for Adults, has captivated readers ever since he appeared in the pages of The New Yorker. Few immigrants have prepared themselves for the responsibilities of citizenship with such ebullience as Hyman Kaplan, and few have played such havoc with the English language. As he tackles his assignments in speech, recitation, and composition, Mr. Kaplan convulses his classmates and sorely tempts his teacher to promote him prematurely. But by final examination time it becomes clear that in the case of Hyman Kaplan, good will can triumph over grammar.