to be используем когда говорим:
1. Я кто? Это что?
I am a teacher. He is a fireman. We are customers.
It is a book. They are pencils.
2. Я какой? Оно какое?
I am cool. He is smart. We are lazy.
It is new. They are sharp.
3. Я где? Оно где? (местонахождение)
I am here. He is at home. We are in the garden.
It is in my bag. They are on the table.
4. Это чьё?
It is mine. They are yours.
5. Сколько стоит?
It is 5 rubles. They are 10 dollars.
6. Сколько времени?
It is five to six. It is a quarter past ten.
7. Сколько лет (возраст)?
I am twenty. He is 5 years old. We are 10.
It is many years old.
8. Во временах Continuous (Я что делаю в конкретный момент времени?)
I am learning English. He is working. We are doing nothing.
9. Перед инфинитивами когда действие запланировано или есть договоренность.
He is to come here tomorrow. We are to meet soon.
It is to be read by all the students.