When the noun ends in a consonant, you will use "은".
When the noun ends in a vowel, you will use "는".
Here are two examples:-
- 물 : 물 + 은 ---> 물은
- 우유 : 우유 + 는 ---> 우유는
I'll now give you some example sentences, including their English translation:-
- 냉면이 쌉니다. 불고기는 비쌉니다 --- Naeng-myun is cheap. However, Bulgogi is expensive.
- 버스가 느립니다. 지하철은 빠릅니다 --- The bus is slow. However, the subway train is fast.
- 여름이 덥습니다. 겨울은 춥습니다 --- It's hot in summer. However, it's cold in winter.