Note that -은/는 is called the topic particle and placed after a noun to make it the topic under discussion. Using the topic particle is one way to designate a noun as the subject of a particular sentence.
1. When a conversation can go any way, the topic marking particle is used to bring attention to what the speaker wishes to talk about. When nothing is being said by either party in a conversation, and a speaker wishes to bring up a topic to talk about, the topic marking particle is used to introduce a new topic to a conversation.
2. If a topic has already been introduced into the conversation, it is old information. If it is old information, it is no longer necessary to mention the topic again (topics and subjects may be dropped in sentences). But if the speaker wishes to clarify what is being spoken about, the topic marking particle is used to clarify the topic of the sentence.
3. The topic marking particle may be used to contrast.
1.저는 한국 사람입니다 – jeo-neun hanguk saramimnida.
2.저는 일본 사람입니다 – jeo-neun ilbon saramimnida.
Sentence 1 uses the topic marking particle because it is bringing a new topic of conversation to attention. Sentence 2 uses the topic marking particle to make the contrast that the person is not Korean, but rather Japanese.
는 (neun) is used when the previous syllabic block ends in a vowel [i.e. 저는(jeo-neun), ㅓ is the last vowel; 성주 씨는 (seongjussi-neun), ㅣ is the last vowel; 대구는, (daegu-neun), ㅜ is the last vowel].
은 (eun) is used when the previous syllabic block ends in a consonant [i.e. 이름은 (ireum-eun), ㅁ is the last consonant; 서울은 (Seoul-eun), ㄹ is the last consonant; 부산은 (busan-eun), ㄴ is the last consonant].
(-는 attached to a noun)
I am Yunseol.
(-은 attached to a noun)
Su Kyeong Park is a student.
(-는 attached to another particle)
There are many people in China.