Nobody really seems to have a concrete answer on this things. I came up with this though, is it right?
Subject = use が. Overarching idea. Like if the subject was “food”, the topic would be “sandwiches”
Topic = use は. Something new to the speakers. Within the subject.
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Ok so take everything I’m going to say with caution, since it’s mostly based on personal experience and various (and sometimes contradictory) sources.
は marks the topic. が marks the subject. Often, the subject and the topic are the same, so only は is used, and が is implied. For example “私はケーキをたべた”
But not always.
Let’s take 私が土曜日に東京に行った.
We could mark different words with は, but the nuance will be different. It’s similar to how the word order in English slightly modifies the meaning of the sentence.
For example :
私は土曜日に東京に行った marks the speaker as the topic, so the nuance is “I went to Tokyo Saturday”
土曜日は私が東京に行った marks the date as the topic, so the nuance is “Saturday, I went to Tokyo”
東京は土曜日に私が行った marks the place as the topic, so the nuance is “Tokyo, I went there Saturday”
It’s the opposite.
Basically subject is a person or thing that either does something, or is described with some qualities. Topic on the other hand is something about what we want to deliver our message. For example, “Speaking about food, sandwiches are something that I like”. We talk about food generally and sandwich is described with a quality “is liked by me”.
However, topic doesn’t have to be explicitly stated in every sentence. For example, topic can be obvious from context, already stated before or topic can be occurrence of some actions/events. For example, when we say “Careful, brick is falling”, what we want to deliver isn’t what brick does, but that such situation actually is happening and we need somehow to react to it.