use of the verb TO BE

To be + adjectiv - It tells the status of something or someone or description.

These examples tell us the status of someone

I am tired
We are hungry 

These examples describe someone

She is pretty
We are American  

To be + un nombre -  He says their relationship, profession or similar.

Examples

I am a lawyer
We are friends  

To be + preposition -  It tells us where something or someone is or what its origin.

These examples tell us where is someone or something

I am at home
It is on the table

These examples tell us the origin of something or someone

He's from Mexico
His coffee is from Colombia 

To be + verb - It is used to create verbal constructions.

If we add the present participle (ing) 

I am working
He is running 

If we add the past participle (ed) 

TV is watched too much
Cake is eaten with a fork 

Other Uses of To be - It is used to indicate the time or this time.

To indicate the time

What time is it? 
It is late 

To indicate the weather

It is sunny
Is it windy?