First of all, I'm not Korean teacher so my explanation make you understand this 은는이가 perfectly. Second, I'm not good at English so please excuse me.
I joined italki few days ago and found that people are struggling 은는이가 을를
One tip is if noun doesn't have 받침(final constant) 는/가/를 if it has 은/이/을
Ex) 나는 I am / 지하철은 This subway / 개가 달린다 The dog is running
물이 차다 This water is cold / 생수를 water / 컵을 Cup
Then how about differences between 은는 and 이가?
If you want to emphasize Subject then 이가 is option, however, if you want to focus on other words 은는.
Ex) 그가 빠르다 "HE" is fast / 그는 빠르다 He is "FAST"
철수가 학생이다 "철수" is a student / 영희는 학생이다 영희 is a "STUDENT"
I think most Korean learners are not really struggling with when to use 은 as opposed to 는 but with in which situation to use these particles and which situations to omit them. They have a hard time identifying the difference (and there is a difference) in using 은/는 in place of 이/가.
It seems like most textbooks are introducing the use of these particles really early on. The truth is, it will take a long time for learners to grasp the subtleties on using the particles, or omitting these particles.
There are a lot of sources that explain the uses 은/는, 이/가, 을/를. Here are some sources
talktomeinkorean.com
goBilly (on Youtube)
Motivate Korean (on Youtube)
My suggestion for all Korean learners that are struggling with these concepts (and other particles) are to learn them as best as you can, find some example sentences, and move on. It will take a lot of exposure to grasp the concept of particles so don't get hung up over it.
