은/는 treats the noun as the topic, so I would say it generally puts more focus on the noun than 이/가. In addition, it also has the contrasting effect, setting the subject apart from other cases. 이/가 is often used when responding to external things as opposed to coming from your own desire to speak. For example if an accident happens, it doesn't come from your own thought or intention, so you say 사고가 났다. 이/가 can sometimes sound like specially focusing on the subject, as when someone asks a specific question (XX 씨가 누구세요? 제가 XX입니다), but I think this can also be thought of as a case of describing/responding to external circumstances - the asker has a need to know and the answeer is responding to this need. 이/가 is also common in sub-clauses because the subject in the sub-clause is usually not the most important point, For example, in 그 일이 터졌을 떄 나는 출장 중이었다, the important main clause has 는 and the sub-clause has 이. Likewise, the key point in "내가 네 말을 어떻게 전부 기억하냐?" is "어떻게 전부 기억하냐?". 나 and 네 말 are kind of a constant, whereas 어떻게 전부 기억하냐? talks about a specific thing. You use 이/가 for this kind of supporting elements without special nuance.