は is used to set the topic of a sentence. It doesn't mark the grammatical subject of the sentence, but rather sets a context. - ピザはおいしいね？ - 私はおいしくないよ。 Translation: - As for pizza, (it's) delicious, isn't it? - As for me, (it (pizza) is) not delicious, you know. In this sample the first person sets the topic to "pizza". This carries on through the whole conversation (or until someone changes the topic). When the next person says 私はおいしくない you might think they're saying "I am not delicious", but "pizza" is still the main subject. は is very hard to translate, especially if you want to make it sound natural. "As for ..." is probably the best one I heard although it doesn't always work. が on the other hand is (at least in this context) used to set the grammatical subject of a sentence. The reply in the above conversation could have been more explicit by saying 私はピザがおいしくないよ。 It was not necessary however. You very often skip the subject in Japanese. It is often used when you want to put emphasis on who or what is/does something. Like if someone says 誰ですか？ you could reply 私がですよ。