~이예요 is equivalent to the linking verbs: is, are, am.
~이예요 is always placed at the end of the sentence.
[Noun] + 이예요 = It is a [Noun].
You can simply place a noun in front of ~이예요 to create sentences as follows:
It is a ~
These / Those are ~
She / He is a ~
I am a ~
You are a ~
책이예요. It is a book. (책 = book)
물이예요. It's water. (물 = water)
선물이예요. It's a gift. (선물 = gift, present)
학생이예요. I am a student. or She/He is a student. (학생 = student)
Now, if ~이예요 is like a linking verb in English and you place a noun you are indicating in front, where is the subject?
Well, in Korean, the subjects are often omitted and can be inferred from the contexts.
So, to be very precise, the above examples can actually be translated as follows.
책이예요. Is a book.
물이예요. Is water.
선물이예요. Is a gift.
학생이예요. Am a student. (or) Is a student.
[Noun (ending with a consonant)] + 이예요.
[Noun (ending wit a vowel)] + 예요.
When the noun has a final consonant (which is called 받침 in Korean), just like:
ㄱ in 책
ㄹ in 물 or 선물
ㅇ in 학생
you add ~이예요.
When a noun ends with a vowel, however, you add ~예요.
강아지예요. Is a puppy. (강아지 = puppy)
맥주예요. Is beer. (맥주 = beer)
여자친구예요. Is (my) girlfriend. (여자 = girl, 친구 = friend)