(Chưa hoàn chỉnh và sẽ còn viết thêm)
The Null-Subject
Some languages are termed null-subject languages in linguistics, which doesn't really mean they haven't got subjects, it means that their subjects aren't always visible, which is exactly what we have in Japanese.
indeterminate subject
Now, there are cases where we have an indeterminate subject, and some people might try to argue that these at least are our mystical subjectless Japanese sentences. But actually they're no different from their English equivalents.
So we might say _(zeroが)_ 晴れそうだ, which means _(_**it**_)_ looks like being fine.
We can't see it, we can't hear it, but we know for a fact that it's there, because without it だ makes no sense at all.