"This restaurant does not sell portions for one person."
Translation:이 식당은 일 인분은 안 팔아요.
The 은/는 particle can be put in place of other particles in order to put emphasis on given word. It's quite confusing, because it has not been introduced in this course. Anyways, the sentence 식당은 일인분은 안 팔아요 means that "the restaurant doesn't sell precisely portions for one person. This indirectly implies, that it does sell different portions, but not the one person ones.
There is nothing wrong with sentences such as "나는 그 책은 읽어요" or "내가 오늘 학교는 안 가요" (는 replaces 를 and 에 respectively). The first one simply means that "I am reading THAT book" (not some random book but some very particular book).
Imagine a situation with two people talking about some book. A third person approaches them, overhears the conversation and says "Oh, I am just reading THAT book (that book you were talking about)".
The next sentence means "I am not going just to the school". The person is probbably going somewhere, but precisely not to the school.
Please, correct me if I am wrong with something.
이 식당(이)는 1 인분(을)은 안 팔아요 =
이 식당은 1 인분을은 안 팔아요
This is called "marker obscuration/ellipsis". The markers are there, just hidden from sight (as not to overburden the sentence). This happens often when the role of the related word in the sentence is obvious.
은/는 are just tags indicating topics for possible further discussion. They do not replace markers although they are often misinterpreted as such (*especially when the markers are 'obscured', as in this example).
There might be a cultural implication hidden in the sentence. Generally, people in Korea don't eat single servings by themselves. They eat family style, with all the food in the middle of the table and everyone around just taking what they want from each dish. I think that's what the sentence is referring to, but I see your point.
Have seen excellent feedback from the 'community'. Here is my contribution to your query:
인분 [ 인 person; (부)분 portion ] stands for "-person portion". It is sometimes simplified to just "인" ( person/head )
When used in the catering and restaurant industries, it is called "-person serving" and is only used with hanja (sino-korean) numbers.
"일 인"분 = "one-person" serving / serving for one. Also written as 1인분.
"이 인"분 = "two-person" serving / serving for two. 2인분.
"Serving for one" usually means a portion deemed adequate by the restaurant for one person's consumption, and not necessarily a portion to be eaten by one person only. In that sense,
Serving for one = single portion (basic unit portion) = 1인분
Serving for two = double portion = 2인분