Particle 는~은

I was a little confused on the function of these particles compared to the 이~가.  I thought 는 ~ 은 are object marker but it turns out to be a topic marker as well (not limited to such function as compared to 이~가 which highlights a noun).  It really takes serious reading to understand each lesson in Elementary Korean.  Its not about reading fast.  The mind needs to absorb.

So what difference does particles 는 ~은 have against the first subject particle I wrote about weeks ago?  Without looking at my book I remember 3:

1. It is used to mark subject that is an old information already.  So If ‘I’ is the subject of my first sentence it will be written as 제가 or 내 (나 + 이).  When ‘I’ becomes a subject again of my suceeding sentence it is written as 제는 or 나는.  I remeber when I was wrote to a language exchange partner, I attempted to write sentences in Hangul by just looking at the dictionary and phrase books.  My sentences where simple then something like ‘I want to be good in Hangul’ and ‘I will try my best’.  At that time he told me that 내, 나, 나는 means the same, it all means ‘I’ or ‘me’.  Thinking that it was like synoyms in English. However in my second sentence he said that instead of 내 it would be better to use 나는.  Not dwelling on this much, i thought its probably just the better choice or the less formal one.  But, thanks to my Elementary Korean book, it is now clearer to me how to choose the correct form for every noun or pronoun that is subject in a sentence.
2. It is a topic marker.  I am a little confused on this.  The book says any word in the sentence can be made topic by putting this particle after the word except for the verb which usually comes at the end of the sentence.   It is said that less important words or information in a Korean sentence is placed first in the sentence and in most cases these are the information that can be dropped.  How does it differ then from rule number 1?
3. It is used to mark direct contrast of topic in a sentence.  I need to practice this more.  Like in the statements ‘I am an employee my sister though is still a student. ‘   So this can be said ‘나는 희산원이에요. 여동생은 학생이에요.”

I am looking forward for the 를 ~ 을 explanation.