>>2290365Not him, but people really overcomplicate this and I'm not sure why nobody can concisely explain it. There are some nuances to it, but it's best summarized as follows.
は is actually fairly limited in its use. All it does is simply mark the topic of the sentence and acts as a relay of sorts for continuing specific aspects of discussion. You can stick entire sentences before は for instance, since its only function is to give focus on a specific thing. This is why people get tripped up on it being a stand-in for a subject particle, since to western Indo-European languages, they are coterminous. However, that specific job of marking a subject is explicitly for が and が alone. Not only that, but if the subject is incredibly obvious or doesn't need reiteration, you simply don't need to repeat は and can also omit the topic (note, NOT the subject) from speech or writing. By that virtue, は highlights things that are more subtle or might need additional nuance while setting the table for discussion, but it cannot introduce new topics either. Whatever it is bound to, it has to be a known aspect of some kind and if not readily known, has to be prefaced on its own. A popular example why Japanese language learners get caught up on this is because 私は is frequently used as a stand-in for referring to yourself. The thing is, a lot of teachers fail to note that the reason why that tends to be a couplet is because, well, you're already obviously existing, right? 私は doesn't introduce anything new because you are already implicitly the topic AND subject of the sentence if you were to bring it up - it's our native language grammar rules tricking us into thinking we constantly need to bring up a marker for ourselves like we do in saying "I, he, she, etc." in practically every sentence. However, if you are talking about an aspect of yourself that may not be known, you'll frequently hear 私のことは - because you are introducing something about yourself that is known to you and may or may not be implicitly known by someone else; the important part is that if you are sharing NEW information that someone else clearly wouldn't know, that's where が would come in.
が is very much the champ at what he does. He's the lad that senses things, he's always the subject, he's always asking questions, he's never going to two-time you. He can be nestled in a は statement or left on his lonesome or even omitted entirely if it's obvious what's happening. He always makes it abundantly clear that he is the subject, and while the subject can be dropped if it's obvious, he can't be mistaken for anything else (even if we're talking formal conjunctions like ですが, since that still carries an implicit subject, or even in casual speech where you might hear 誰が instead of 誰か .) He can introduce new information, sensations, events, everything and anything. The problem is, he can come off as brusque and simple, and often without nuance if you overuse him. He can't carry too much since he is simply a marker for the subject. That's why the topic is there to help soften things up and introduce more information, because if there's one thing the Japanese language hates, it's repetition of particles outside of lists.
I hope that clears it up for you.