some time ago, i wrote about using WA/GA, and how an article helped me to understand it.
after reading some more about the subject, it seems that it’s a bit more complicated.
now, i am really, really not that good in japanese, so the following is only how i understood the rules. they might be completely wrong. the examples were taken from sci.lang.japan
generally it seems that you do
so, for example, in the following dialogue:
>A: What is Bob? >B: Bob is an idiot.
the second line is translated using WA:
>B: Bobu WA baka desu.
but in this dialogue:
>A: Who is an idiot? >B: Bob is an idiot.
GA is used:
>B: Bobu GA baka desu
for this reason, when there is a longer conversation, it’s usually started with GA, and continued with WA.
some more examples:
>Inu ga heya ni haitte kimashita. >Inu wa boku no gohan o tabete shimaimashita.
>A: Hora, asoko mite. Tanaka-san ga iru. Kare wa eigo no sensei janaika? >B: Tanakasan wa gakusei-da. Tainakasan ga sensei da.
to learn more, search for “WA GA” on the sci.lang.japan group