This paper shows the existence of a pretonic assimilation of *y to a following coronal consonant (including *y from proto-Semitic *y and *w) in North-West Semitic languages. This rule, which has been obscured by analogy in each of the North-West Semitic languages, explains three independent sets of facts: the formation of irregular maqtal-s in Hebrew, Phoenician and Aramaic; the irregular conjugations of several verbs in Hebrew; and the plural formation of the irregular noun "house" in Hebrew and Aramaic. This proposal also solves the long-standing problem of the etymology of the verb "to give" in North-West Semitic languages (NTN in Hebrew vs. YTN in Phoenician).