I agree with Chris. It's not an easy one to understand. Using "wa" is fairly easy. You place "wa" after the subject of a sentence which simply states what the subect does or is: Watashi-wa gakusei desu. (I am a student.) Kare-wa ongakuka desu. (He is a musician.) Anata-wa nihonjin desuka? (Are you Japanese?) Takeshi-kun-wa kabin-wo kowashi mashita. (Takeshi broke a vase.) Otousan-wa shinbun-wo yonde imasu. (Father is reading a newspaper.) If you replace "wa" with "ga" in the above sentences, each subject remains as a subject of the sentence, but it puts emphasis on the subject. Very often, it implies that there might be other choices for the subject. Watashi-ga gakusei-desu. - It is I who is a student. This sentence would be an answer to a question such as "Which one of you is student, you or he?" Kare-ga ongakuka desu. - It is he who is a musician. This sentence would be an answer to a question such as "Is there a musician in this room?" Anata-ga nihonjin desuka? - Is it you who is Japanese? An answer to this question could be "hai, watashi-ga nihon-jin desu." Takeshi-kun-ga kabin-wo kowashi mashita. - It is Takeshi who broke the vase. This would be an answer to a question such as "Who broke the vase?" "ga" is also used after a subject of a clause in a compound sentence: Takeshi-kun-ga kabin-wo waruno-wo mimashita. - I saw that Takeshi broke the vase. "ga" is also used after a subject of a descriptive sentence as a result of observation: For an example, a teacher asked students to describe what they see in a picture. A student who saw an image of a father reading a newspaper would respond, "Otousan-ga shinbun-wo yonde iasu". For the most, we use "wo" after an object. "ga" or"wa" after an object place an emphasis on the object, and slightly change the implication of a sentence. Watashi-wa neko-wo katte imasu. - I have a cat as a pet. Anata-ga sukina doubutsu-wa nan desuka? - What is the animal that you like? Watashi-wa neko-ga suki desu. - I like cats (puts emphasis on cats). Anata-wa neko-ga suki desuka? - Do you like cats? Watashi-wa neko-wa kirai desu. - I don't like cats (but I like other animals). Anata-wa nNani-wo benkyou shite imasuka? - Wat are you studying? Watashi-wa suugaku-wo benkyou shite imasu. - I'm studying math. Watashi-wa suugaku-ga nigate desu. - I am not good at math (puts emphasis on math). Watashi-wa suugaku-wa nigate desu. - I am not good at math (but I am good at all other sujbects). I hope this helps.

I've been learning Japanese for 3 or 4 years now, and I still don't fully understand the difference. I think it's one of the most difficult aspects of the language and takes many years to fully comprehend. If you're a beginner, it's probably best just to try to understand the basic difference between marking topics with "wa" and marking subjects with "ga" (you should be able to find textbooks or websites that provide an introductory account of this), and then the best you can do is hope to develop your understanding as you learn more vocabulary and more complex grammatical structures. After you've got the basic differences clear in your mind, I'd recommend looking at Jay Rubin's "Making Sense of Japanese", which has a very good section on "wa" and "ga" (and also one of the best explanations of "giving" and "receiving" verbs that I've come across). The only problem is, your Japanese needs to be better-than-basic in order to understand the example sentences used in this book (I'm still struggling with it, but I find it's a useful struggle nonetheless!). Overall, I think it's futile to try to find an explanation of "wa" and "ga" that will tell you everything you need to know at once. The usage of these 2 particles is just too complex, and has to be picked up gradually over time. (At least, that's what I keep telling myself, in order not to abandon all hope of ever reaching fluency!) I hope this answers helps, and is not too discouraging!

HA designates TOPIC. Topic is a new concept for anyone learning Japanese, but can be uncerstood with some practice. The reason for this is that the topic is not grammatically displayed in the English language. But it is there if you look well enough. I suggest you get some practice by locating topics in English sentences. GA designates the subject of the verb! Watashi HA kami GA nagai. = My hair is long. (The topic is "me") Kochira HA atsui! = It's hot over here. (The topic is "here") Just keep on practicing. It will become clear.