There are three types of ga-ellipsis in Modern Standard Japanese: Syntactic Ellipsis, which optionally deletes a nominative case particle in syntax, Lexical Ellipsis, which obligatorily deletes a nominative case particle that immediately follows one of the designated lexical items, and Dialogic Ellipsis, which optionally deletes a nominative case particle marking a first- or second-person subject of the matrix clause. A nominative case particle cannot be deleted by Syntactic Ellipsis or Lexical Ellipsis when the expression marked by the particle has a thematically lower clausemate. Likewise, a nominative case particle cannot be deleted when it marks a focused expression.
(S. Yatabe, "Gendai Nihongo ni okeru san-shurui no shukaku joshi shôryaku genshô [Three types of ga-ellipsis in Modern Standard Japanese]," in Takao Gunji, ed., Seiyaku ni Motozuku Nihongo no Kôzô no Kenkyû [Constraint-Based Studies on the Structure of Japanese], Kokusai Nihon Bunka Kenkyû Sentâ (The International Research Center for Japanese Studies), Kyoto, 1996, pp. 223-239.)