은/는 is used when you talk about a subject that you thought or felt about on your own or - i.e. something you have an opinion or feeling about. It sets the noun as the focus of the sentence.
As a side effect, it can also have the effect of contrasting the noun against others, like "as for" or "speaking of (the thing)".
- 나는 영화 보기를 좋아한다. (something you're familiar with)
- 내 동생은 고기는 좋아하지만 생선은 싫어한다 (like meat but not fish - contrasts the two).
이/가 usually describes something that is occurring independent of your own thoughts, like when you notice something. Anything that happens independent of what you had on your mind can be marked with 이/가.
- 저 사람이 나를 쳐다봐. (That person is staring at me - external, unexpected thing).
- 선생님이 내게 질문을 하셨다. (same)
So 은/는 is generally for well thought out things or general truth, while 이/가 suits unusual or unexpected things, or a particular or unusual instance of something as opposed to the general case.
- 화재는 무서운 것이다 = Fire is a scary thing (statement of general truth - 는)
- 우리 동네에서 화재가 발생했다 = Fire broke out in our neighborhood (a specific instance of fire - 가)
- 하늘은 푸르다 = The sky is blue (general statement - 은)
- 하늘이 노랗게 보인다 = The sky looks yellow (specific, unusual incident - 이)
That's a simplified introduction. It is a subject too big to be explained in a couple of paragraphs.