[hitokiri679]

While the info on that page is basically correct, I do not like the way he teaches these particles at all.

This is definitely a topic you'll have to come back to multiple times before you understand every nuance, but I'll try to help you understand the most basic points.

First, the particles が and を are never interchangeable. At the most basic, が marks the subject of a verb, を is for the direct object, and a third particle, に, marks the indirect object. So in "I kicked the ball to him", "I" is the subject, "the ball" is the direct object, and "to him" is the indirect object. In English, only the indirect object has a preposition, but in Japanese every one of these is marked with a particle.

Each of these also has other uses, but they are unrelated to the places where は is also involved.

は itself has two basic meanings - it marks a "topic", or it marks contrast. In Japanese, the topic of a sentence is often the same as the subject, in which case it overrides が. It's also possible for the direct object to be a topic (this creates emphasis on the object), in which case を is replaced with は. If you understand this much, you're ready to start tackling the difference between subject and topic and get into the practical differences between は and が.

To answer your question, not quite. は is generally used whenever you simply want to make a "comment", that is, present (new) information about the topic, which by necessity is someone/something *the speaker and listener must know the identity of*.

So you've got it a bit backwards. One of the places where you revert to が is where the subject is a question word.

ex. だれが　きますか。 / Dare ga kimasu ka? / *Who* will come?

In this case, the listener might know who "dare" is, but the speaker certainly doesn't, so it is a subject, but not a topic.

In more complex sentences, there are other reasons why you might use は or が, and because the subject is not necessarily the same as the topic, you can also have both in the same sentence.

I know I really skimped on what makes a topic, but please tell me if any of that doesn't make sense.