Membership awareness to differentiate は and が?

Question

So, I've been trying to properly use は and が. It is often said, online, to differentiate using topic and subject. That has always confused me, so I noticed a way to use it that seems to always follow the following rule: use が if someone is unaware of the thing you're talking about, and は if everyone is aware.

For example, you would only say "there's a car" if you know someone was unaware of the fact, therefore, "車があります" is used.

Alternatively, "the car is green" is used when everyone is aware of what car you're talking about, therefore, "車は緑です" is used.

Is this a functional guideline, or are there glaring exceptions that I have not thought of?

Answer 1

While what I am going to say will not directly answer your question, I think it might help so I decided to post an answer.

The link posted by Pleiades above as a comment has a pretty good description of the various causes where は and が are used. The only problem is that there is still a lot of vagueness, and depending on your skill level in Japanese trying to memorize all of the uses is very tricky.

What I have done to help refine my understanding of が and は is whenever I am reading, I am very cognizant of which of these is being used, and I will stop to think deeply about it. If I can't fit it into a rule I know, I just try to remember that specific situation. I also recommend writing a log of sentences which have uncertain は/が usage, and you can add notes to them at a later point and compare and contrast different scenarios.

Ultimately, unless you are going to be a Japanese teacher, what I think is most important is not whether you have a list of rules (though that helps), rather it is an intuitive understanding. Eventually your mind learns to find the patterns itself, even if you cannot put them into neat and tidy rules.