This study investigates plural noun forms in Modern Hebrew to show that the pattern of regularization differs for specific words, rather than for classes of words. Early regularization, in which the children add a plural suffix with no stem change, applies unconditionally. Subsequently, the pattern changes and while some words are still regularized by six year olds, others are given the correct plural form by grade-school age children. Other words continue to deviate from the norms of classical Hebrew, even in the speech of educated adults. A test was administered to 48 native Hebrew speakers of high socio-economic status, 12 each at the ages of three, six, and twelve, and non language-major college students. The 20 items tested plural formation for two classes of feminine nouns and two classes of masculine nouns. Results reveal the complex interaction between formal linguistic knowledge and incidental knowledge due to familiarity of specific forms in language usage and in formulaic contexts. Both kinds of knowledge are developmentally determined, yet some historically deviant forms have entered standard usage. What constitutes an error may need to be evaluated differentially with respect to the language of young children compared with that of older speakers. (Author/JK)