-이다

이다 is the copula 'to be': attached to a noun, it says what something is (X is Y). It conjugates like an adjective into every speech style.

문장 끝·말투Endings & speech style

Han: After a consonant the past is 이었어요; 였어요 is only for nouns ending in a vowel (친구였어요).

Examples

제 동생은 고등학생이고 저는 회사원입니다.My younger brother is a high school student, and I am an office worker.

오늘은 나의 스무 번째 생일이다.Today is my twentieth birthday.

어제는 일요일이라서 가게가 문을 닫았어요.Yesterday was Sunday, so the shop was closed.

-이다vs있다있다 means 'exist, be located, have'; 이다 states identity: 교실에 학생이 있다 'There is a student in the classroom' vs 그는 학생이다 'He is a student.'

N+이다 (after a vowel 이 often contracts or drops: N + 다)

학생 → 학생이다, 학생입니다, 학생이에요, 학생이었다; 의사 → 의사(이)다, 의사입니다, 의사예요, 의사였다

It is written attached to the noun with no space (학생이다, not 학생 이다) and is used only after nouns, pronouns and question words, never after verb or adjective stems.
After a vowel 이 contracts: 의사예요, 친구였어요, in plain style 친구다. After a consonant it stays: 학생이에요, 학생이었어요.
Before a noun it becomes 인 (학생인 동생 'my sibling who is a student'); to link clauses it becomes 이고, (이)라서 or 인데.
The negative is not 안 + 이다 but N이/가 아니다: 학생이 아니다.

Wrong: ~~어제는 제 생일이였어요.~~

Right: 어제는 제 생일이었어요.

After a consonant the past is 이었어요; 였어요 is only for nouns ending in a vowel (친구였어요).

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