This paper analyzes the grammaticalization of English modals. The comparison of the diachronic versions of the Bible clearly shows that the grammaticalization of English modals has been developing gradually and not abruptly. To explain the gradual stages of the grammaticalization of modals within generative grammar, I propose that parameter-resetting of diachronic changes is a result of the change of the status of markedness, especially concerning English modals. As the rate of occurrence of modals as lexical verbs and auxiliaries overlaps and crosses each other, the parameter-resetting occurs, which changes the status of the unmarkedness of the modals as lexical verbs to marked lexical verbs. In Shin (1992; 2006; 2019), I find evidence of English reflexivization, null-subject parameter including impersonal verbs, and the change of English word order. Another evidence is that of Fischer et al. (2000:17―8). After the date of the assumed parameter-resetting of the inherent case’s availability in the second half of the thirteenth century, examples of adjective followed by a PP can still be found, side by side with the former adjective + object. A universal evidence besides English is that of the present-day Chinese yào, which functions either as a lexical verb “want” or an auxiliary verb “will,” like the OE willan.