Функции глагола to be:

1.

Местонахождение чего-то и где-то - Moscow is in

Russia. Pen is on the table.

2.

Долженствование (если после to be стоит

частица to) - I am to go there. He is to do this work

fast.

3.

Профессия или обязанности - Alex is a Captain.

John is a father. Mary is a student.

4.

Выражение состояния или эмоций

человека. - I am busy now. He is angry at me. My

mother is a lucky person.

5.

Замена тире или слово "это" - Diamond is a

mineral. It is clear.

6.

Подстраховка отсутствующего глагола

- We are friends. The weather is good.

7.

Участие во всех формах времени Continuous

(Progressive) - I am speaking now for you.

We are going to Miami tomorrow.

8.

Пассивная форма глагола -  Passive voice

- I am invited. He is killed (rewarded).