hey 아케미 I am so sorry, to be honest I actually droped your diary earlier but I gave up my comments ㅡㅡ;;damn it. How silly of me. hahah I think I can do it. it will take about some times to do it.
what I meant is I don`t think you will understand in my comments.
If I tell you simply. the best way is listening to it a lot reading a lot.
that`s it. haha Oh! here is the bottom line is use it practically a lot in conversation, if your partener ask you " Kieth 는 키가 어떠니 " like th you just should reply " keith는 키가 커요 " that`s it. try to use them a lot. if another weird sentnece comes up, don`t have to be confused.just listen, and read. don`t memorize. just let it be. got it ??
Think about it 은는이가 are not diffcult compare to " The , " a " in English.

Hi, Akemi, In the Korean grammar, it is one of the most difficult problems to distinct and use correctly 은/는 and 이/가. Even the best grammarians in Korea may have difficulty to explain it clearly. As you may know, in Japanese, we have the same difficulty in explaining the distinction and usage of 'は' and 'が'.

As Keith writes, you will learn the correct usage of them in reading and writing a lot of Korean sentences.

Here I answer you one thing. As you wrote, one of the typical explanations is that 이/가 shows the subject of a sentence. But in this case, you're right, the sentence doesn't necessarily depict 'the action of the subject'. 이/가 is also used to show the subject of a sentence that is a simple description of something.

꽃이 아름답다. The flower is beautiful.
산이 높다. The mountain is high.

It's hard to explain.Actually many children is writing default like my english. haha.Especially in english.

이/가 is a main subject's fuction.은/는 is like a assistance of sub.
(*mark means a fault)

(1) 이가 is used at first time. ‘옛날에 한 나무꾼[青]이[/青] 살았습니다. 그 나무꾼[青]은[/青] 어느 날 나무를 팔러 시장에 갔습니다.’
결국 ‘은/는’is used when you know it already or it is mentioned already. next two sentences is changed 이가 and 은는.
한 나무꾼이 살았습니다. 그 나무꾼은 나무를 팔러 떠났습니다.
*한 나무꾼은 살았습니다. 그 나무꾼이 나무를 팔러 떠났습니다.

(2) ‘은/는’is used when there is the special one in same group.
‘철수가 집에 있다’ this sentence says the simple state of a fact.
‘*철수는 집에 있다’ this sentence means 철수is different with others.

(3) Generally ‘은/는’is used, when you wanna explain something. 은는 is come behind a object's word.
인간은 생각하는 동물이다.
*인간이 생각하는 동물이다.

But the sentence focus on a object's word. we use 이가
‘이 유리를 누가 깼니?’ >> ‘철수가 깼어요’
this short conversation is interested in who broke the window glasses.
이 유리를 누가 깼니? / 철수가 깼어요.
이 유리를 누가 깼니? / *철수는 깼어요.

(4)은는is used by a main S. 이가 is used by a verb dependanted.
나는 그가 왔음을 안다.
*내가 그는 왔음을 안다.
there is two verb. come and know. it is simple in eglish.
"I know that he comes." Main verb is 'know'

:P sorry~~
Maybe it is hard..Explain was sucks.;;
and I don't know well about it..;;
But it's the best I can. I did the best..^^;;