‎what’s the difference between using 은/는 or 이/가 ? there are no subject or topic marking particles in english so it is hard to figure out which word to use it on and which one to use in the sentence

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'은/는' is mainly used to define someone or something.
while '이/가' simply explains or describe someone/thing's current status or movement.

ex) 그는 의사이다. He is a doctor.
ex) 그가 움직이고 있다. He is moving.
ex) 지반이 흔들리고 있다. The ground is shaking.


And here's some difference when it comes to daily conversation.

1.오빠는 의사이다.
2.오빠가 의사이다. both means "My big brother is a doctor."

In the first sentece '는' is soley being used to define and explain about who he is.
It usually requres the other sentence, syntax to complete the second sentence.
for example,

"오빠가 의사이다. 그러니 병에 걸려도 걱정 없다."
My big brother is a doctor, so I'm not worried about getting sick.

or it can be used when you refer to some hidden messages in the conversation,
especially when those who having the conversation understands what it means to define what is what using "이/가". For instance,

A : "결혼 잘했네. 돈 걱정은 없겠어." ⇒ Good for her, not much to worry about finance.
B : "왜?" ⇒ But why?
A : "그 애 남편이 의사거든." ⇒ Her husband is a doctor.

I know it's complicated, you can use 은 instead of 이 in sentences from above and it will be gramatically correct, but the 이 sounds more native.


And here's some examples of the situation you must use "이/가"

나는 영화가 정말 좋아. I like movies so much.
너는 책이 좋니? Do you like books?

실험보단 이론이 먼저야. Theory must come before experiment.


Hope it answers to your question.
If it's too complicated, just focus to the first two sentences I wrote in the top.