In Tsutsui (2006) (in JLL 40), I claimed that da in "A wa/ga B da" does not have a copula function, and argued for it based on observations of the presence and absence of da and its variations (da+ hereafter) in various structures, including "modal sentences" such as "Taro wa isha rashii," noun modification structures such as "Ani ga isha no Taro," conjunctive structures such as "Taro ga isha ni-mo-kakawarazu," and adjective predicates such as "Taro wa genki da" and "Taro wa tsuyoi {∅/*da}." I also presented the hypotheses that the functions of da+ are: (1) that of marking a tenseless predicate as a non-past or past, making the surface form of the tenseless sentence or clause complete, and (2) that of indicating the politeness and formality level of the sentence/clause to which it is affixed; and that da must be present when a sentence/clause needs explicit tense or politeness-level information, and must not be present otherwise. I argued for these hypotheses, demonstrating that the seemingly odd distributions of da+ in various structures including those mentioned above can be systematically explained without introducing any ad hoc da-deletion rule.
In this paper, accepting the view that da+ does not have a copulative function, I will review the ways da+ and issues related to this element are taught in the JSL context, and suggest alternative ways of teaching them. Specifically, I will propose that we not teach da+ as the copula (e.g., as the device to link A and B in "A wa/ga B da" or the equivalent of "be") but rather explain that A and B are linked structurally. I will also suggest that we not introduce i-adjectives and na-adjectives together, but rather introduce na-adjectives and nouns together, explaining that na-adjectives are a kind of nouns with an adjective property.