There have been a number of studies about the roles of the particles “wa” and “ga” (Mikami 1953,1963, Kuno 1973 among others). The results of these investigations are that a noun marked with wa indicates the theme of the sentence or the contrast and that a noun marked with ga indicates exhaustive listing or the neutral description (Kuno 1973).
This paper proposes that wa is in reality two different particles, one that marks topic (wa1) and the other, contrastive focus (wa2) whose NP is somehow a topic but has marked focus (Van valin 1997). These two, being discourse markers, occupy specifier or adjunction of TopP (wa1) or Foc P (wa2) which are higher positions than IP (Rizzi 1997). By the same token, ga also represents two different particles; one, a focus marker which marks exhaustive listing (ga1) has a discourse function, but the other, which marks “neutral description” (ga2) is a nominative case marker. While the discourse marker ga1 occupies specifier of FocP, the nominative case marker ga2 occupies the specifier of IP. According to this analysis, sentences which have multiple ga and are said to have two subjects (Hasegawa 1999) in a sentence turn out to have just one subject because the other(s) do not have explicitly grammatical functions.