This article reports on how, against a background of relatively stable patterns of second language negation, a Japanese-speaking adult learning English made use of a negative formula, "I don't know," and how, in and through interaction, analyzed it into its component parts and began using "don't" more productively. Making use of the micro-analytic techniques of conversation analysis to analyze data collected over a seven-month period, two relatively stable patterns of negation are described. This is followed by a description of how the learner used the formula and, over time, analyzed it. This often involved repetition and/or self-repair. Changes in how "don't" was used included coming to use it with the verb "like," as well as coming to use it with "you."