I have just studied Korean for a short time. I am having problem with the sentence below.
당신은 이름이 뭐예요? = What is your name?
My friend told me that there are two subjects in that sentence (당신 marked by 은 and 이름 marked by 이). I am confused now. Should it have only one subject, your name?
Also which is the Korean equivalent word of "your" in that sentence?
There are different ways to analyse the grammar, but one common way employed is to say that this sentence has a Topic - Comment structure.
In a topic - comment sentence, the topic does not necessarily need to be an argument of the verb (it can, but doesn't need to). What it does is to establish the context of the sentence. Then the comment is a syntactically correct sentence with a predicate, what makes sense in the context that the topic provides.
In Korean, the topic of the sentence is marked with the ending -는. Now, in Korean it is possible for that topic to also be an argument of the verb - to be a subject or object, for instance - and in fact, in most cases, the topic is also the subject - but it does not have to be. It can be, in a sense, syntactically independent. As some answers explain it, in school grammar the topic is considered the subject of the comment, which in its whole is called a predicate. I think this is perhaps a way of analysing it so that the topic is not independent; either analysis works well.
So in the sentence 당신은 이름이 뭐예요?, the comment is just 이름이 뭐예요?, meaning "what is the name?" This doesn't make sense on it's own, without any context - but the topic of the sentence, 당신은, provides the context. We are talking about "you" (당신), and so the question 이름이 뭬예요? provides context - not merely "what is the name", but "for you, what is the name?" - that is, "what is your name?"
We can see other examples of where the topic - comment structure is used with an independent topic:
철수는 머리가 좋지 않다 (Literally: Cheolsu, head isn't good; which means "Charles isn't too smart"). The context (Cheolsu) makes the comment (head isn't good) make sense.
나는 비행기 타는 것이 무섭다 (Literally: I, riding a plane is scary; which means "I'm afraid of flying in a plane".
영희는 계산이 빠르다 (Literally "Yeonghee, calculation is quick"; which means "Yeonghee is good at numbers/calculation".
S 사업은 문제가 많다 (Literally "S Company, problems are many"; which means "S Company has many problems".
This is how the sentence is explained in Korean school grammar:
이름이 뭐예요 forms a single predicate clause, and the subject of the predicate clause is taken by 당신은. That means there are two (nested) clauses in this sentence,
이름이 뭐예요
당신은 (이름이 뭐예요)
Then the clauses get to avoid the two-subject dilemma, since technically, the only subjects in each of the clauses are 이름이 and 당신은.
This works for all sentences which have seemingly two subjects; except the case where 되다/아니다 are predicates.
This is not analyzed as two clauses. Rather, 내가 is the subject of 되었다, and 선생님이 is the complement of 되었다. 되다/아니다 are exceptions because they always require two sentence elements, i.e. 선생님이 되었다 does not make sense.
I don't know how modern grammarians interpret similar sentences, but when I learned Korean grammar in middle school, they were called 서술절 ("verb clauses"). It is not found in English or (I think) many other languages: basically you have a whole clause with its own subject and verb, and then the clause itself acts like a verb for the outer subject. For example:
(The part in [...] acts like a verb for the subject outside of it.)
성층권은 [온도가 낮다]. The stratosphere has low temperature.
Literally: The stratosphere [the temperature is low].
지수는 [배가 아팠다]. Jisoo had a stomachache.
Literally: Jisoo [the stomach was hurting].
[사람들이 그렇게 많던] 광장이 이제 고요하다. The plaza, which had so many people, is now silent.
Literally: The plaza which [people were many] is now silent.
You can even have multiple layers:
철수는 [표정이 [변화가 없다]]. Chulsoo doesn't change his face much (i.e., he has a "poker face").
BTW, if you are a novice it is OK, but "당신은 이름이 뭐예요?" may sound a bit impolite (since you're apparently talking to a stranger!). As a rule of thumb, try not to use the word "당신": unlike "you", there are very few situations where the word "당신" is appropriate, and simply dropping it is usually better. (Yeah, I know it would be difficult. It's often difficult for us Koreans. Sorry about that.)
A better expression might be "이름이 어떻게 되세요?" or "성함이 어떻게 되세요?"
