In this study, I have collected diary entries written in Imperial Japanese by a cohort of Japanese individuals who belonged to the generation that experienced secondary education during the Japanese colonial period. The aim of this study was to examine the characteristics of the language acquisition phase by using these texts as illustrative examples. Furthermore, I conducted an analysis by comparing the findings with discourse data from the Japanese contemporary generation, which shares similar characteristics and is currently undergoing the process of Japanese language attrition. The results can be summarized as follows: (1) In the secondary school generation of Japanese speakers, standard forms of existences, potential expressions, aspectual expressions, and negations were used more frequently during the acquisition phase than during the attrition phase. (2) Dialectal forms and non-standard expressions were also present during the acquisition phase, but their frequency gradually increased as the duration of the attrition phase increased. This phenomenon can be interpreted as indicating not only failures in language acquisition, but also regression after the acquisition process. (3) In terms of specific linguistic forms, evidence of the learner's restructuring strategy can be seen in the use of existences and aspectual expressions, while a general simplification process associated with second language attrition can be observed in the reduction of negations. Additionally, transfer factors are influential in the use of potential expressions. Nevertheless, there may be individual differences. For future research, it is crucial to expand the diary data set and to continue with comparative analyses of Japanese generational groups who have undergone elementary education.