Ok. 은/는 doesn't talk about someone exactly but 이/가 talks about someone or more than just one person. 이/가 exactly means "the" in English. Like you say "give me the book" you are talking about a book that you know and you mentioned that and you want to get 'the' book. In this situation we use 이/가. But if you say "give me a book" you don't talk about a mentioned book. You just want a book without knowing that. Usually we use 은/는
I don't know if you've found the answer to this yet, but it is pronounced like "가치". This is because sometimes the ending consonant in a syllable interacts with the first letter of the next causing one of them to change. In this case a ㅌ at the end followed by a 이 makes a 치 sound.