"The singer is uncool."
It’s official. The pronunciation of ㅅ in 멋없다 is ㄷ. Naver shows the pronunciation as [머덥따]. This is also the case for the similar-sounding 맛없다 (tasteless) which is pronounced [마덥따].
TL;DR: 멋없다 is read as a compound word that just so happens to be written without a space because arbitrary rules; the ㅅ does not carry over to the next syllable. 멋있다 is not perceived to be a compound word, but one unit, so it’s pronounced (nearly 100% of the time) as [머싣따]; the ㅅ carries over to the next syllable.
멋없다 is actually a compound of 멋 and 없다. Both are morphemes. 멋이, 멋을, and 멋은, however, are pronounced respectively as 머시, 머슬, and 머슨 because 이, 을, and 은 are not morphemes but markers; 멋이 is a word with a subject marker—not a compound word.
The common pronunciation of 멋있다 as [머싣따] is due to its popularity and not technically correct. But dictionaries acknowledge the popular pronunciation as well, and you will almost never hear (correct) [머딛따] uttered in speech.
It is connected to the end of the adjective as if it were "uncoolnotis", so look for 멋없습니다
Actually, it's the other way around: 이/가 is used for the subject and 은/는 is used for the topic. What that basically means is that 은/는 has the implication that you are comparing the topic of the sentence to something else, or that you will continue talking about it. 이/가, on the other hand, has no such implication, so it is used more often in simple sentences whereas 은/는 is more useful for full narrations. However, the difference between them is very slight so they are mostly interchangeable.
Example: 차가 비싸요. 그런데, 가방은 싸요. The car is expensive. But, the bag is cheap. Because you are comparing two items here, you could use either particle in the first sentence, but you must use 은/는 in the second sentence.
It's really hard, and I am by no means an expert, but that's just what I've been taught in my Korean class. Hopefully, that helps!
Also, sidenote: you should almost always use 저는 when talking about yourself.
Even with the explanation, I am still not sure. https://www.duolingo.com/skill/ko/basics-1/tips-and-notes
Is this beginner vocabulary? REALLY? If so you really need to split the course in 2: Korean for BTS fans ; Korean for normal people :) Seriously - we've got "The singer is unpopular" "The man is uncool" " Books are meaningless" etc... ALL before "Where is the bathroom?" Giant face-palm