The two particles every Korean sentence uses: 은/는 marks the topic or a contrast, 이/가 marks the subject.
은
Topic particle after a consonant ("as for...")
는
Topic particle after a vowel
이
Subject particle after a consonant
가
Subject particle after a vowel
저는
I (as for me)
제가
I (subject form of 저)
나는
I (casual)
내가
I (casual subject form)
저는 존이에요
I'm John
제가 할게요
I'll do it
누가 왔어요?
Who came? (question words take 이/가)
친구가 왔어요
My friend came
이름이 뭐예요?
What's your name?
날씨가 좋아요
The weather is nice
비가 와요
It's raining
가방이 무거워요
The bag is heavy
커피는 좋아해요
Coffee, I do like (contrast: but maybe not other things)
고기는 안 먹어요
Meat, I don't eat (contrast)
오늘은 바빠요
Today I'm busy (as for today)
서울은 커요
Seoul is big
한국어가 어려워요
Korean is difficult
한국어는 재미있어요
Korean is fun (as a topic)
머리가 아파요
I have a headache (my head hurts)
저는 키가 커요
I'm tall (as for me, my height is big)
동생은 학생이에요
My younger sibling is a student
이것이 제 거예요
This one is mine
이게
This (subject, short for 이것이)
이건
This (topic, short for 이것은)
여기가 어디예요?
Where is this place?
시간이 있어요
I have time
사과가 맛있어요
The apples are tasty
형은 의사이고 저는 학생이에요
My older brother is a doctor and I'm a student (male speaker)
버스가 와요
The bus is coming
저는 서울에 살아요
I live in Seoul
카페가 어디에 있어요?
Where is the café?
문이 열렸어요
The door is open (it has opened)
꽃이 예뻐요
The flowers are pretty
제 동생이 아파요
My younger sibling is sick
누가 한국 사람이에요?
Who is Korean?
그 사람이 누구예요?
Who is that person?