이/가 is for indicating subject of sentence when the subject is a specific name or animal or people.
There is a term '주격조사' in Korean grammar, which means word for indicating a subject of sentence.
(For reference, you can use '에서' for indicating subject when it is a inanimate thing, for example, company, organization, region, area.)
The main usage of 이/가/에서 is '주격조사'. And there's also term for representing a word that functions as adding special meaning of sentence by following right after other words, that is called 보조사. 이/가 have subsidiary function as 보조사, and give a special meaning to words by indicating theme of sentence.
While you should attach '이', '가' to other nouns when they are used as '주격조사' or '보조사', '이' can stand alone in the sentence when '이' itself is used as a pronoun, but some context can help to know what the pronoun indicates is required when you use it.
Grammatical term of 은/는 is 보조사. As abovementioned, 은/는 gives additional meaning of sentence since they are 보조사 in Korean grammar, indicating a theme or contrasting one thing with another.
은/는 can be placed at the subject place or object place of sentence. Quite frequently, words with 은 becomes subject when they are theme of sentence.