있다 / 없다 — the existence pair
있다 (itda) means 'to exist / to have / to be located somewhere'. Its opposite is 없다 (eopda) — 'not to exist / not to have'. In polite speech you'll say 있어요 and 없어요. These two verbs do triple duty: they tell you a thing exists, that someone has it, and where it is. The pattern is short: [noun] + 있어요/없어요. To say where something is, slap the location particle 에 onto the place: 집에 있어요 (it's at home).
Existence: 'There is X'
Bare and simple: 책이 있어요 = 'there is a book' (or 'I have a book' — same words, context decides). 시간이 없어요 = 'there's no time'. The subject usually wears the subject particle 이 (after consonants) or 가 (after vowels). Don't confuse it with the topic particle 은/는 from lesson 2 — for now, plain existence statements take 이/가.
Possession: 'I have X'
Korean doesn't use a separate verb for 'have'. To say 'I have a book', you say 'as for me, a book exists' — 저는 책이 있어요. The owner is the topic, the thing owned is the subject of 있어요. 저는 시간이 없어요 = 'I don't have time'. 친구가 있어요? = 'Do you have a friend?' (raising intonation makes it a question — no extra word needed).
Location with 에
에 is the static-location particle. Glue it onto the place: 집에 (at home), 학교에 (at school), 한국에 (in Korea). The pattern is [place]에 있어요: 저는 집에 있어요 = 'I'm at home'. Note that 에 is also used for destinations (where you're going to), but that's a later lesson — at A1, just learn it as 'at/in [a place]'.
Where is X? — 어디에 있어요?
어디 = where. To ask 'Where is the book?' say 책이 어디에 있어요? Word order: subject + question word + 있어요. The answer is the same shape with the location filled in: 책이 집에 있어요 = 'The book is at home'. Korean keeps the verb at the end whether the sentence is a question or an answer.
Polite 계세요 for honored people
When the existence/location is about a person you respect (a parent, teacher, customer, elder), Korean swaps 있어요 for the honorific 계세요 (gyeseyo). 선생님이 학교에 계세요 = 'The teacher is at school' (polite). Don't use 계세요 about yourself — that would be self-honorific and very weird. For now, just recognize the form when you see it.
