이다. 나는 학생이다. i'm a boy. : description. ~(noun/pronoun)이/가 있다. 여기에 책상이 있다. There is a desk here. : existence. ~(verb)하고 있다. 나는 공부하고 있다. I'm studying. : present progressive

They're two very distinct words, so there really aren't any "nuances" between the two, but instead two very different meanings and usages. It's a common beginner's question. No hate. Just saying :) In short, 이다 is used to equate one thing to another (like an equal sign). 있다 is used to show existence. These are two very different things. Saying "There's a mouse in my room" ("There is" shows existence) is much different than saying "My room is a mouse". They are two very different verbs. "I have a cat" ("have" shows existence) is a lot different than saying "I am a cat." ("am" shows equality... I = a cat)

The Verb'이다' : to be '이다' is the form which links a subject with its predicate, indicating equality or identification. If this form is attached to the noun, there can be no pause or space between it and the Noun. It is pronounced like a part of the Noun. It has two different forms '-예요' and '-이에요'. '-예요' is used when the Noun ends with a vowel, and '-이에요' is used when the Noun ends with a consonant. 안나 + -예요 --> 안나예요. 책상 + -이에요 --> 책상이에요. The verb `있다' indicates existence, location or possession. The opposite verb is `없다'. (Possession) 동생 있어요? Do you have a younger brother? 네,동생이 있어요. Yes, I have a younger brother. 아니오, 동생이 없어요. 그런데 언니는 있어요. No, I don't have a younger brother. But I have an older sister. (Location) 교실에 책상이 있어요? Is there a desk in the classroom? 네, 책상이 있어요. Yes, there is a desk (in the classroom) 아니오, 책상이 없어요. No, there is not a desk (in the classroom). 의자는 있어요. But there is a chair.