Wa vs. ga
The ONLY purpose of "が" is to mark the subject before it, thus placing emphasis on it, answering such questions as "Who? What? When? Where?", to which the answers would be an emphasized "I (am the one who...), THAT (is the one which...), NOW (is the time when...), THERE (is the place where...)", etc, even if that question was only implied and not necessarily asked!!!! For example if I were to ask you "だれが 行きました？" (WHO is the one who went?); if it was you, you'd say "I am the one who went.", with emphasis on the "I" (私が いきました。) , noting that emphasis with the use of が, the particle that marks the subject before it, thus emphasizing it. If you're introducing someone or something, obviously, the subject isn't known, so with the use of the subject emphasizer が, you answer an implied question like "Who is the person who lived a long, long time ago in a galaxy far, far away?" by, say, beginning a story, like, "LUKE SKYWALKER is the one who lived a long, long time ago in a galaxy far, far away." and your character is now introduced so you can make him into a topic (は) for further discussion. [REMEMBER: が = English "A (person named Luke Skywalker)" (used when subject isn't known, so you have to introduce it), and は = English "The (guy named Luke Skywalker that I just introduced to you and you now know of.)"
は, on the other hand, NEVER marks the subject, that's strictly が's job. Yes, that's right, I said は NEVER marks the subject, and I'll tell you why in a little bit. は, contrary to が, simply differentiates what you want to have a conversation about from other possible things you could talk about. Of course, this thing you're having a conversation about will always be known by both the speaker and listener because you can't have a conversation about something the listener doesn't even know about yet (if he doesn't, that's what が is for, to clear up who/what/when/where for the listener, even if he didn't specifically ask it. That's because only YOU know what you're going start a conversation about, and it's up to you to introduce that topic to the listener with が if he doesn't know about if already, you don't wait for him to ask. は ALSO places emphasis on what comes after it whether it be a verb, adjective, etc. IT COULD BE ANYTHING! は shoots your attention to what comes next like,"Me? I went to town and SHOPPED." answering an implied question like,"Now let's shift our focus to you, Bob. What is it that you did?" Why did I say that は NEVER marks the subject, only が does? Surely,"これは ペンです。" is modifying これ, right? WRONG! Why? Because what you're really saying is,"As for this,well, IT is a pen." In Japanese, the invisible word "IT" is eliminated because the topic of これ is already known - it would be redundant to repeat it again. Thus ぺんです was ACTUALLY modifying the invisible word "it"(which, if visible, would be marked by the subject marker, が, NOT は！ Therefore, が retains its reputation of being the subject marker, with は retaining its only function - marking the TOPIC, not subject, and shifting the focus ahead to the invisible pronoun, marked by が because that's the actual subject of the sentence, so you know that invisible pronoun is what is being described. That's right, EVERY sentence has [subject]が in it, it's just that it's sometimes hidden. Thus, the day is saved and all is well again. THE END.
[IF YOU DIDN'T CATCH THAT, HERE ARE THE KEY POINTS:]
--> Marks subject before it, thus placing emphasis on it. --> Answers implied questioning of the subject when introducing something. (ONLY が CAN INTRODUCE! NOT は!) --> When you're introducing something in english, you use "([NameHere]is) A (person/animal/thing,etc.)", so therefore, が, being the introducer, is the equivalent of the English word "a".
--> Marks what the topic of conversation is about, which is already known or has been introduced with が. --> When something is known in English, you use "THE (person/animal/thing,etc.)[did such and such/is something])", so は, being the marker of a known topic, is the equivalent of the English word "the". --> Places emphasis on WHAT ABOUT the person/animal/thing,etc. that comes late in the sentence. --> THIS IS THE BIG ONE! "Him?(は) *HE* went skiing." What's skiing modifying? NOT "Him", BUT "*HE*", THE SUBJECT, WHICH WOULD BE HIDDEN
IN THE JAPANESE VERSION OF THAT SENTENCE BUT WOULD REALLY JUST BE PRESENT AND MARKED BY が, JUST INVISIBLE!
(I wrote this before I knew about this website, so that's why it sounds so informal and unprofessional.)