My greatest bit of advice would be...don't stress~! "은/는 & 이/가 are very difficult grammatical concepts for most learners to grasp. Study the grammar, but don't stress to much about which to use. Over time, as you read and listen to native materials and practice speaking and writing and getting corrections, you will get a feel for which is right in a situation. Additionally, some words or grammar constructs use ONLY or almost always 이/가." -Me There are many resources out there for this if you do some digging. This is an age-old question asked a million times by many learners. So age-old that I can't manage to find any of my own old answers in my feed >.<

To name a few; 저는 ( used if topic of the stament is me.) 오늘은 ( used if the topic of the statement is today) 징구가 ( friend is the subject of the statement ) 사과가 (apple is the subject) 잭 이(book is the subject)

Check the comments :)