This paper discusses three issues on the errors and non-uses of learners: (1) How should we grasp the mechanism of errors? (2) What should we do with the reality of errors? (3) How should we consider the difference of views on errors? On the issue (1), it is claimed as follows: (4) Every error has a natural reason for why it occurred. (5) Error and non-use are two sides of a same coin; they occur as a result of the failure of acquisition of expressions in contrast. On the issue (2), it is claimed as follows: (6) Errors should be taken up positively; they tell us the points of learning rathar than bother us. (7) One cannot do without grammatical items of elementary level when expressing oneself. To improve the learners' ability of speaking (writing), a close reexamination is needed of syllabi of these items. (8) Errors which do not lead to misunderstanding should not be taken sensitively. On the issue (3), it is claimed as follows: (9) Error analysis and contrastive analysis are not overcome by the study of second language acquisition; there still remain the importance of error analysis and the significance of facts of language transfer. (10) It is important to consider what to do with learners' language instead of merely studying how it is.