Well, today you're getting some grammar because you need it. You're going to learn how to say "A is B" (where A and B are nouns) in polite Korean.
Now, one thing we have to make clear first is that Korean is a bit of a pain when it comes to sentence structure. Whereas in English we follow a S-V-O (Subject-Verb-Object) pattern i.e I kick the ball, in Korean they use S-O-V pattern i.e I the ball kick.
이에요 (i-e-yo) / 예요 (ye-yo) have a similar role to the English verb 'to be'.
In English, the verb is changed according to the subject (and also the tense - but we will discuss that later in the collection) for example; I am, you are, he/she is. In Korean, while the verb does change according to the tense, it does not change according to the subject but rather according to whether the last letter in the previous word ended in a consonant or a vowel.
Final Consonant + 이에요
Vowel + 예요
사과예요. (사과+예요) = It's an apple.
제 친구예요. (제 친구+예요) = It's my friend.
책이에요. (책 + 이에요) =It's a book.
By raising the tone at the end of the sentence, it can also be used to make a question.
뭐예요?= What is it?