I was watching Misa’s video on ある & いる + particles and she says when using ある it’s usually paired with the particle が. Hence the structure: object がある. Why is this so? From my limited knowledge, the only usage of the が particle I know is to put emphasis on a subject or identify something.
You know how it would be weird to say “I’m going to the party” if the person you’re talking to doesn’t know about the party? That’s because using “the” implies that the speaker expects the listener to already know about what follows it (which is the “party,” in this example).
The は particle in Japanese is similar to English’s “the” in this sense: because は is used to provide context and redirect the conversation to a new topic, it is weird to shift the topic into something that your listener doesn’t know anything about.
So, to use は properly, we should be using it only on things that our listener is familiar with.
Therefore, if we want to introduce something new to the listener, for example… what even exists… we use が instead. Once something has been introduced to the listener with が, it can be correctly referenced later with は.
(Also, I suspect that this is why it’s said that が can be used to emphasize what comes before it; if our listener is already familiar with something, meaning we can safely use は on it, yet we still choose to use が in spite of that, it shifts some extra attention onto whatever this particle is marking.)