Resolved questions
What's the difference? 2

1) Between 은/는 and 이/가
이/가.: subject case particle
은/는: topic particle
Eg:
1.사라는 한국 사람이 아닙니다.(Sara is a Korean)
"사라" is not the subject but the topic of the sentence. Notice that the subject of the sentence is omitted, and 한국 사람 , is the subject of the sentence

2. 친구가 세 영이 왔어요.(Three friends came)
Notice that there are two subjects in the sentence, 친구 and 세 명 as
both are marked by the subject particle. Korean grammarians call such a sentence “double-subject construction.” Double-subject sentences are very
common in Korean. However, its interpretation is not that the sentence
has two subjects. In this sentence, the focus is on the number three rather
than friends.

This contrasts with the subject marked by the particle 이/가The subject
particle 이/가 is used to mark a subject (which happens to be new information
or has not been mentioned previously in the context). For instance, this
explains why most interrogative words such as 누구“who,” 무엇“what,”
언제 “when,” and 어느 “which,” are used with the particle 이/가, as in
누구(가), and 무엇 (이), but not with the topic particle 은.는
어느 식당 음식은 맛있어요 “Which restaurant food is delicious?” --> 틀려요

어느 식당 음식이 맛있어요“Which restaurant food is delicious?” --> 맞아요
누구는 미국사람이에요? “Who is an American?” (X)
누가 미국사람이에요? “Who is an American?” (O)
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2) Between 미안합니다, 죄송합니다, and 송구합니다. And why isn't 송구합니다 used frequently these days?
송구합니다, may be this word is out of date
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3) Between 어머니/아버지 and 어머님/아버님

when 님 is added, it turns into "horrific form". Korean ppl often call parents in law by adding 님

Hope it can help!