>I have read that が can be used when something new is introduced to a sentece: >https://www.learn-japanese-adventure.com/japanese-particles-wa-ga.html It is an explanation for beginners, which cannot cover all. Now you are running into an uncovered area. In my opinion. --------------- ”は" originally means "emphasize", which leads to "topic". ”が" originally means the "owner" of a noun, which is widened to use as 1).the "owner" of verb or adjective, and 2). conjunction. (Note that I don't use the term "subject"). --------------- I believe this definition covers all (but not for beginners) OK: 1). ある日、息子が山で働いていると、やぶの中からしらがの鬼ババが出てきました。 One day when the son was working in the mounttain, ONI-baba came up from.. NG: 2).ある日、息子は、山で働いていると、やぶの中からしらがの鬼ババが出てきました。 Sentence #2 does not make sense, because Theme: ある日、息子は、 Rhema, what is said about Thema 山で働いていると、やぶの中からしらがの鬼ババが出てきました This rhema does not correspond to the theme.

It's probably because he's not the topic, just the grammatical subject in the sentence, ie: we're not actually talking about him, we're just saying that he was working there at that time..