You add 는 to emphasize on the noun. So, if you literally translate this sentence into English it would be, "As for Korea, you cannot wear shoes in the house." There are two usages of 는. First as a topic marker and second as to compare or to generalise something. Here you are saying, "As for Korea (regardless of what rule is in other countries), you cannot wear shoes". So you are just stating a general statement. I hope this explanation helps:)
(1) @dipagssi. Nouns can be turned into adjectives (modifiers) but not with 은/는. This usually is done by using special affixes s.a. 적(이다) ; 스러운 ... e.g. 역사, history; 역사적인 = historic/ historical or 사랑, love; 사랑스러운, loveable
(2) -는 used here is a topic marker as both you & SadiaA explained very well. And it can carry the contrasting undertone.
Yeah, you get yelled at if you are familiar to the person owning the house. You'll get politely asked to take them off if you are some kind of honored guest. And to top it off, there are floor drains in a lot of the bathrooms, so sometimes the bathroom floors are wet. So there are "common" rubber slippers in a lot of bathrooms for you to put on so you don't get your socks wet. Oddly enough, it's not polite to go barefoot in someone else's house either. So if you're a girl used to wearing heels barefoot you either don't or bring socks or hope they have some closed toe slippers for you wherever you're going. And there are certain restaurants where you sit on the floor (and therefore you take your shoe off ... you see the same here in the States in Japanese restaurants). And, strangely, sitting on the floor is considered a more formal dining experience than on chairs. And in Korea that is the main reason you don't wear shoes indoors ... you end up sitting and many time sleeping on the floor.
In Tunisia (north Africa ) we can stay in the house with the shoes. Some people (some not all) stay in their shoes until it's time to sleep so they charge their clothes and shoes. Some people stay in their shoes if they have to go out again. But if there is no need to go out they change them.
-(으)면 안 되다 ( = should not; must not ) is used to express a prohibition.
Its opposite is:
-(아/어/여)야 되다/하다 ( = should; must; have to ) is used to express permission, obligation
-(으)면 되다 does exist but it is used to express a weak form of advice (more of a suggestion/recommendation ). It is akin to = could; may
yo the word bank is well confusing on this one - i don't know if it's because i'm british but sometimes I struggle with the English I'm supposed to say to some of these