I totally agree with jon.^^/ It's not easy for Korean learners, even advanced learners to tell the differences between the subject particles,i.e. 은/는(topic particles) and 이/가(identifier particles) However, Here is what I think. 1.I ate the apple. 내가 사과를 먹었어요.(not other persons but I) We are interested in "WHO" with the identifier marker 가. It can be an answer of "WHO ate the apple?" 2.I ate the apple. 나는 사과를 먹었어요. This sentence with the topic marker 는 can put an emphasis on "사과" or "먹었어요" It could be an answer of "WHAT did you eat for lunch?" or "What did you DO?" (We already know you did something, just want to know what happened, so the subjects are often left out) "제가" and "저는" are the polite forms of "내가" and "나는" repectively. 3.When we are talking about general facts, we use "은/는" It's quite similar to why the present tense is used to describe general things in English. 하늘//은// 파랗다 = the sky is blue(general fact) 하늘//이// 파랗다 = the sky I saw yesterday was red, but it's blue now (talking about the specific color of the sky) The giraffe has a long neck = 기린//은// 목이 길어요.(we know it's true) = 기린//이// 목이 길어요(X) (This sentence would be correct if you found a mutant^^/) Water freezes at 0℃. = 물//은// 영도에서 얼어요.(general fact) = 물//이// 영도에서 얼어요(X) (it would be correct in other planets^^/) \^o^/

It feels like this is the most common question when it comes to Korean. Listen to this lesson and you might understand a little bit more. Then google "는 이/가 difference" and you will find a lot of info. And finally, be prepared that it will take a long time before you understand how to use them correctly. I can't even get it right all the time and I have studied Korean for a year. The audio lesson: http://www.talktomeinkorean.com/lessons/l1l9/

The way I was taught at the Defense Language Institute was to think of it this way: 내가 김치 먹어요 -I eat kimchi 나는 김치 먹어요 -As for me, I'm eating kimchi

i really want to help you! But..나도 잘 모르겠어요.