"여자는 인기가 없습니다."
Translation:The woman is not popular.
When "marking" nouns with 은/는, you can translate it like "As for ___,".
"여자는 한국 사람입니다." may better be translated as "As for the woman, she is Korean." It COULD also be translated as "As for a woman, she is Korean." The former makes much more sense in context.
I could confuse you more by saying "As for women, they are Korean." is also a valid translation, depending on context.
However, the point here is not to confuse you with definiteness (the vs a/some) nor number. If anything, this should relieve you. The 은/는 particle just marks something that you want to focus on. It's a new noun that is introduced to the conversation; a new topic to talk about. You have to think like a Korean at this point. When you hear a noun, think of that noun in context (collectively, singularly, plurally, etc. depending). Maybe it's difficult now, but with practice and an open mind, you will soon understand.
The difference between 은/는 and 이/가: 이/가 is a particle that marks a subject or noun that is not NECESSARILY the topic of the conversation. It may be, but most of the time, it's just a mentioned noun.
"여자는 한국 사람입니다." and "여자가 한곡 사람입니다." could be translated the same in English, but there's a huge difference in nuance for Koreans when they hear these two sentences!
The sentence with 여자는 suggests that they start talking about (the/some/all) woman ("as for (the/some/all) woman,") as a topic, while the sentence with 여자가 doesn't necessarily suggest that they start talking about (the/some/all) woman as a topic; rather, it just mentions (the/some/all) woman doing/being (or NOT doing/being) something. When you hear a noun followed by 은/는, your brain should realize that that noun is the topic of the conversation, the emphasis and/or focus of the sentence. When you hear a noun followed by 이/가, you should listen to the sentence, but you can also shrug it off because it's just a mention of some noun and it's being/doing (or not being/doing).