when do I use "dass" and when do I use "das"

"Das" is an article that is almost always followed by a noun. Das Pferd, Das Buch. If it is not followed by a noun it usually has become the noun. Das geht nicht. Das ist richtig. The easiest rule to determine whether das is spelled with one s ore two is to substitute das/dass with "dieser", "jener", "welcher" if the sentence still makes sense after having been changed, das will be spelled with one s. If the sentence doesn't make sense anymore das will be spelled with 2 s. Also, before dass with 2 s there is ALWAYS a comma.