This paper attempts to analyze the semantic function of the Japanese case marker ga and introduce the notion of a zero particle option.
Two types of ga are identified according to their semantic functions, i.e., marked and unmarked. The marked ga functions to stress the information in the immediately preceding NP. The function of the unmarked ga is one of neutrality. In some cases, the unmarked ga becomes marked due to the syntactical shift of a sentence, for example, from an affirmative sentence to an interrogative one. When such a syntactical shift occurs, sentences in which the speaker does not intend to focus on anything more than simple questioning and in which elements related to the addressee's abilities, feelings or sense of possession are contained produce a marked ga. In such sentences, topicalization neutralizes the markedness of ga.
Based on these observations, it is hypothesized that when a sentence contains elements which may in some way potentially affect the function of the sentence (herein called 'functional crossover'), one function becomes the most dominant in terms of the meaning conveyed by the sentence. It is possible that other grammatical phenomenon may be explained by this Functional Crossover Principle.
Lastly, the implications for Japanese language teaching are suggested. The distinction between particle deletion and zero-particle option is made. Focus is placed on the validity of including zero particle as a grammatical option along with marked and unmarked ga.