이다 is the basic Korean grammar form for 이에요/입니다/입니까. It is used with a noun to make it predicate of the sentence. 이다 is basically just used to express the subject and predicate of the sentence. Also 이다 can be used to simply specify something.
The polite form of 이다 is 입니다.
저는 제임스입니다. (I am James)
네, 책상입니다. (Yes, its a desk)
The question form is:
그 분이 제임스입니까? ( is that person James)
책상입니까? (Is this a desk)
The above are used polite speaking so use the above when speaking to Koreans,just when you are in the basic stage. It is easy to offend people in Korean ^^. Here are the less polite words, but they are also just generally used in everyday talking.
This grammar point can be used when specifying something or asking a question about something. Just by adding a “?” to the end of it, the sentence becomes a question.
이에요 (This is used when attached to a noun that ends with a constant)
예요 (This is used when attached to a noun that ends in a vowel)
a) 책상이에요? (is this a desk?)
b) 네, 책상이에요. (yes, its a desk.)
a) 사과예요?. (Its a apple)
b) 네, 사과예요?. (yes its an apple)
The negative form of 이다 is 아니다