I just read an article explaining that が should not be thought of as a subject particle, since it can also mark the object, which is fine and dandy on its own.
However, I also remember reading an article that explained that when certain transitive verbs (e.g. 話す) get conjugated into particular forms (e.g. potential), anything that would have been marked with を is instead marked with が.
At first, these two bits of information seemed contradictory (the fact that the latter was explained in terms of the object becoming the subject didn't help). In trying to reconcile them, however, I came to the conclusion that を can be replaced with が (with a change in nuance), but が can't (necessarily) be replaced with を. Does this seem accurate, or is one of the sources of information wrong?