Hahaha. They’re tricky. Basically though, I think there are three main frames of reference we can use to figure out which is more natural, which give us a total of seven or eight rules (depending on whether you want to count frame #2 as one or two rules):
- What each tends to emphasise (this is a classic of Japanese teaching): が emphasises what comes before it. は emphasises what comes after it.
- What each tends to be used for, at least in a case without special emphasis:
- が for affirmation, は for negation and questions
- が for verb phrases, は for ‘[noun] is [noun/adjective]’ statements
- The five main differences, which you can find here (I did a summary in the original post if you need it, so don’t bother expanding the translation unless you want all the details. It’s not my translation, by the way):
Source article from ARC Academy here in Japanese
(If I feel like it some time, I might get round to translating that article myself and posting the translation somewhere on the forums instead of constantly directing people towards that well-intentioned but frankly rather awkward translation. Guess I’ll see how I feel about that…)