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

• [interesting] GA [something]
• [something] WA [interesting]

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