Roughly speaking, WA is a topic marker and GA is a subject marker. WA is used to mark something which has already introducedinto the conversation,or is familiar with both a speaker and a listener. GA is used when a situation or happening is just noticed or newly introduced. This is a basic rule. WA is also used for contrast. Watashi wa nihonnjin de, Tsukinowa wa irakujin desu. GA is also used with question words. Nani ga suki desuka? - Manga ga suki desu. GA is also used for Emphasis. Kenji ga ikimasu. ( Kenji does go.) Hope this helps you : )

Categorization of Wa and Ga and Phonetic Prominence Phonetic prominence influences the meaning of sentences. Consider its interaction with wa and ga in a very simple example sentence. In (3) are examples which include wa. +F indicates phonetic prominence. (3) a. (Jon wa kimashita ka?) Ee, Jon+F wa kimashita. (Contrastive) John TOP came Q Yes John TOP came (Did John come?) Yes, John came. b. (Jon wa kimashita ka?) Ee, Jon wa+F kimashita. (Contrastive) c. (Jon wa kimashita ka?) Ee, Jon wa kimashita+F. (Thematic) The prominence in (3c) seems weaker than those in (3a-b). (3a-c) are all affirmative answers to "Did John come?," but they are different in nuance. (3a-b) imply that someone other than John, say Mary, did not come, while (3c) does not have such an implication. (4) illustrates examples of ga. (4) a. (Dare ga kimashita ka?) Jon+F ga kimashita. (Exhaustive) who SUBJ came Q John SUBJ came (Who came?) John came. b. (Meari wa kimashita ka?) Iie, Jon ga+F kimashita. (Exhaustive) Mary TOP came Q No John SUBJ came (Did Mary come?) No, John came. c. (Jon wa kiteimasu ka?) Iie . . . a, Jon ga kimashita+F. (Descriptive) John TOP has-come Q No oh John SUBJ has-come (Has John come?) No . . . oh, John has come.

The usage of wa and ga is determined by discourse rather than single-sentence syntax or semantics in many cases. It depends on the information structure of sentences. In (1a), kore (this) is old information, and watashi no (mine) is new information. Conversely in (1b), kore is new, and watashi no is old. (1) a. Kore wa watashi no desu. this TOP mine is This is mine. (Ans. to "Whose is this?") b. Kore ga watashi no desu. this SUBJ mine is This is mine. (Ans. to "Which is yours?") However, their usage is more complicated. Wa and ga are categorized as in (2) (Kuno, 1973). (2) a. wa i. thematic wa Kujira wa honyu-dobutsu desu. whale TOP mammal is A whale is a mammal. ii. contrastive wa Jon wa sono hon o yonda ga, Meari wa yomanakatta. John TOP that book OBJ read but Mary TOP didn't-read John read the book, but Mary didn't. b. ga i. descriptive ga Tegami ga kita. Letter SUBJ came Mail has come. ii. exhaustive-listing ga Jon ga gakusei desu. John SUBJ student is John (and only John) is a student.