The verb 'be' can be used in different ways. Here are some common ones:
(A) 'Be' and its forms are usually used as linking verbs, linking the subject with a following word.
(B) 'Be' is used as a full verb by itself in the sense of 'exist'.
(C) 'Be' is also used to express a command or request in the sense of 'become'.
Example for (A)
1. Sampath is a lawyer. (linking the subject with a Noun)
2. Sumathi is very intelligent. (linking with an Adjectives)
3. My car was in the garage. (linking with an Adverbial Phrase)
4. Govind is to retire next year. (linking with an Infintive)
Example for (B)
1. God is. (exists)
2. "I think, therefore I am". (exist)
Example for (C)
1. Be quiet. Be off.
2. Be a good boy. Be nice to her.