I have read the following sentence:
휴게실에는 누가 있습니까?
However, I don't understand why 는 is required here. Isn't it that 에 is enough to express the location, right? What is the difference between 에 and 에는 here?
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Let's say you have the following conversation:
주방에는 철수가 있고, 식당에는 영희가 있죠? Is Cheolsu in the kitchen and Younghui in the restaurant?
(그럼) 휴게실에는 누가 있죠? (Then) Who (else) is in the lounge?
철수 아버지가 계세요. Cheolsu's father is in the lounge.
As you can note above, the asker knows who are in other two places, but wants to ask specifically who is in the lounge other than the two people.
'는' is a particle that indicates an emphasis for one specific place in which the asker doesn't know who is. As you mentioned, you can omit '는' in the sentence. But the nuance is quite different when you use it.
휴게실에 누가 있습니까? Is there anyone in the lounge? (Who is in the lounge?)
It generally inquires if there is anyone in the lounge. Depending on the tone, the answer could be either "Yes / No" or "Cheolsu is there". It is more likely the former is used for an answer.
휴게실에는 누가 있습니까? Who is in the lounge? (Is there anyone in the lounge?)
It specifically asks who is in the lounge. It doesn't necessarily mean it can't ask if there is anyone in the lounge.
It all depends on context and tone. Hope you see the difference.
The particle 은/는 is sometimes used as an emphasiser. Ultimately, you could argue that this is always what it is being used as.
Although Korean learners are taught (at least I was) that 은/는 is a subject marker, it really is just being used as a marker that indicates emphasis or weight in a sentence.
So, in your example, 휴게실에는 누가 있습니까? is asking who is in the LOUNGE (emphasis on the lounge). The meanings here are rather nuanced and almost entirely vocal when used in English.
I actually think that some Koreans almost use the 은/는 particle as a filler more than a distinct and purposeful piece of the sentence. There are of course times where the usage is purposeful and emphatic, but sometimes Koreans use 은/는 as a mere filler. They probably do not always even have a conscious reason for using it.
Please note that the verb stem ending -은/는 is not the same as the particle (조사) 은/는 I am talking about here.