"The pencil is a thing."
You really have to go by context There is no clear-cut. But basically,
이/가 - used for
¤ introduction (a/an);
¤ indication (the ... there)
¤ generic (a, an/'0'- article);
¤ referential (the aforementioned);
¤ contrast (a; the; 0-article);
¤ emphasis (a/an; the; 0-article)
"저" is the first-person pronoun as "I" in English.
"은/는" are the Korean topic-marking particles.
When the noun ends with a vowel, attach "는" to the noun;
When the noun ends with a consonant, attach "은" to the noun.
- "입니다" is a conjugated form of "이다" (to be). Here it serves as one of the endings for declarative sentences.
I speak japanese and it also has particules similar to those two (general subject/what is concern about the action or emphasis on the subject). And if the particules are used the same way, it should be "un" as you said. // For example = Un : Sam is a man (affirmative) / I : Sam loves animals (as an emphasis) [- Who does love animal? - SAM loves animal] (Emphasis on Sam) // I hope someone could confirm though...
This is called nasalization go watch "korean unnie" video it will help But in case you don't have the chance to do then you need to know that the letters "ㄱ ㄷ ㅂ " when its followed by "ㄴ or ㅁ" its pronouncrd as "ㅁ or ㄴ or ㅇ " depending on the word As we still beginners i think it still too early to go that deep and btw you need to found you some other source for learning as the applications are not very helpful there are a lot of good websites and youtube channels go check em Hope i was helpful