Topic Particle - 은/는

은 and 는 are topic particles. They are attached to nouns, adverbs, other particles or endings. It shows the subject of the sentence or may be used to show "contrast" or "emphasis".

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:-

1. 물 : 물 + 은 ---> 물은
2. 우유 : 우유 + 는 ---> 우유는

I'll now give you some example sentences, including their English translation:-

1. 냉면이 쌉니다. 불고기는 비쌉니다 --- Naeng-myun is cheap. However, Bulgogi is expensive.
2. 버스가 느립니다. 지하철은 빠릅니다 --- The bus is slow. However, the subway train is fast.
3. 여름이 덥습니다. 겨울은 춥습니다 --- It's hot in summer. However, it's cold in winter.