This study investiagted the use of the adverb yoku to modify a verb and proposed a teaching guide for its use and meaning, using unnatural examples from the International Corpus of Japanese as a Second Language (I JAS). The I JAS was created using spoken language collected from native and non-native Japanese speakers’ talking tasks. Based on previous studies’ analyses of yoku, we classified the adverb yoku examples into those connoting degree, frequency, and meaning. When we compared Korean learners’ usages with those of native Japanese speakers, we found that examples of degree were used in sentences expressing an essence (e.g., well) or comparing actions (e.g., better), and examples of frequency were used in sentences referring to accidental occurrences (e.g., much, often). Unnatural examples involving yoku were used to connote degree in sentences that referred to accidental occurrences and not single actions. Unnatural examples involving yoku were also used to connote frequency in sentences with negative predicates. Korean learners used 23 different instances with different meanings such as “many” and “easily.” In addition to these findings, this study suggests some effective ways to teach students about using the adverb yoku.