[lucagalbu]

why do you use が? Doesen't it depend on the context? E.g. 猫は二匹いる　means "there are two cats", while 猫が二匹いる　 means "two cats are there"; i.e. in the first case I focus the attention on the fact that there are 2 cats (and not 3 or 4 or...) while in the second case I focus the attention on the fact that there are cats (and not other kind of animals).
...well, maybe it's not a good example, but I mean: it depends on the context if I use wa or ga, isnt'it?