I’ve been self-studying for the past ~5 months, and I feel like I’ve finally made it somewhere in understanding this age old problem. I haven’t bought any books or participated in any courses during my studies – mostly just immersion – so I can’t really gauge how correct my intuition is on this:
If you already know what you are asking about and you aren’t identifying it, then you can use “wa”.
For example: “ashita ga ame desu ka?” sounds like you’re asking “is TOMORROW the day that it will rain?” instead a of “will it rain tomorrow?”. The reason it sounds weird in English is the exact reason it sounds weird in Japanese. Though I can see it being used in a context where you thought it was going to rain on another day, so you shockedly say: “ashita ga ame desu ka?!” as a reaction. Kind of like “it’s gonna rain TOMORROW?!”.
When you use “ga”, it always sounds like you’re pointing at something and talking specifically about IT. This is why a more correct question would be “itsuhi ga ame desu ka?”, with which you’re specifically asking “on which day will it rain?”. This is also why we use “ga” when talking about what we like and so on, since – for example – it’s responding to the question of “nani ga tabetai desu ka?”, or in English, “WHAT do you want to eat?”.
Another example: “ringo wa oishii desu ka?” – are the apples tasty? We are inquiring about the attributes of the apples, not whether the apples are the ones that are tasty out of some kind of selection. Again, if we wanted to know what exactly the thing that someone likes is, we would ask “nani ga oishii desu ka?” instead.
FINAL EXPLANATION: An unknown subject always takes “ga”. You NEVER use wa after “nani”, “dore”, “docchi”, etc.. This is why answering “neko wa suki desu” to “donna doubutsu ga suki desu ka?” produces the nuance of liking cats, but maybe not other animals, since you’re avoiding identifying (AKA using ga) that it’s simply cats that you like – it’s basically changing the subject.
The nuance of wa excluding something is still a bit blurry to me – nevertheless, I need to hear what more experienced people have to say.
by Lopsided-Good7076