Even though I've read this article, I would still like to clarify something about the 다 particle/suffix (not sure of the terminology, as a beginner).
So far, and as someone with a basic knowledge of Japanese grammar (which allegedly is similar), I understood that the 다 particle plays a role similar to Japanese da/desu in a simple declarative sentence, being da in both languages the declarative particle at the end of the sentence.
In Korean questions, the declarative 다 is replaced by the interrogative particle 카, which reminds of Japanese ka.
As far as I understood, the Korean suffixes -mni, -seumni are basically "decorative" expressions, whose only function is to add formality to the sentence, between a verb root and the declarative 다.
However, in dictionaries and textbooks, I noticed that verbs and adjectives (or rather "descriptive verbs", I'm not sure), when mentioned outside a complete sentence, have 다 appended to it.
For example, 많다 ("to be numerous", or just "numerous"), 없다 ("to be absent", "not to exist"), 아름답다 ("to be beautiful", or just "beautiful").
Even though these are the dictionary forms, surely there are contexts in which the 다 suffix must be dropped, and as such it must not be interpreted as a part of the verb root.
If I understood correctly, when expressing a state-of-being, if the subject "is" another noun, then the verbal root 이 is required, either directly before 다 or before the polite variants -mni-da / -seumni-da. As such, "That man is a teacher" would be "그 사람은 선생 입니다"; the answer to "Who is he?" could be just "선생 입니다" since the subject is droppable in this sentence, and the answer to "Is he a teacher?" could include just the verb: "입니다".
Moreover, some words translated into English as adjectives rather than as verbs work in a sentence with no other verb: thus, "That girl is beautiful" translates as "그 소녀는 아름답다", and "She is beautiful" can be told just as "아름답(습니)다".
Perhaps the 다 after verbs and adjectives on dictionary entries is a reminder that those words (unlike nouns) can stand for a complete sentence with that very particle?