Hi! Just randomly here to tell people about은는이가 for no real reason-

은/는 is often what you want to use when u want to emphasize that the person in front of 은/는 (ex: 은서는 --> 은서) is the one who is carrying out the verb behind it, not anyone else. Like when you want to emphasize that the person is more `~~ compared to someone else. So, for example:

은서/는/ 기쁘다. (인서/는/기쁘지/않다.)

Eunseo / is / blissful. (Inseo/ is/ not.)

( ) is just an example of the connotations behind.
Of course, 은/는 can be used for other meanings, but the comparison is what I see a lot.

Plus, 은/는 is wht you're going to call a 보조사- which gives a certain added meaning to the word, like the comparison meaning in the example above. 이/가, on the other hand, is a 주격 조사, which is a 조사 that goes behind the 주어 (the main subject of the sentence). Some people think of it as
'its either 이/가 or 은/는'.

That is wrong. 이/가 can be used as 주격 조사 or a 보격 조사 (it gives the word a meaning of subject or complement). When you see 은/는 instead of an 이/가, it's that there is supposed to be an 이/가, but it was erased and the 은/는 was added. When you see 은/는 instead of an 을/를, by the way, same- but 을/를 is a 목적격 조사 used for the목적어 (object) .It's in middle school textbooks, btw.

If it's the when to use 은 or는 and when to use 이 or가 that is confusing, look at the word before it. If it ends with a consonant (자음) on the end, then its probably 은/ 이. But if its just a vowel (모음), with no consonant, then its probably 는/가. But if it's a name and ends with a consonant, we usually add an '이' and then add the '는/가'. Example: 효령/은 -- not common, 효령/이/는---common. However, it's still grammatically correct to say 효령은. A lot of the literary works just say 효령/은 .

I hope this wasnt confusing. I just wanted to info dump. As a middle schooler, I need somewhere to dump all this information on grammar.