Verb TO BE

 Verb "TO BE"

The verb "to be" can express two meanings: to be or being. So much to say that I am a teacher (I am a teacher), and to say I'm at school (I am at school), you must use the verb "to be."

The verb "to be" is conjugated in the present "am", "is" and "are". 

Ex.:
You are a student. 
He is my neighbor.
She is at the supermarket. 
The dog is outside.
We are friends. 
You are my enemies.
They are upstairs. 
 
For the negative, just add "not" after the verb "to be" (am, is, are)
They are not in Their bedroom. (They are not in their room).
We are not nurses. (We are not nurses).
I am not your teacher. (I'm not your teacher).

You can abbreviate the verb "to be" with "not". This form is widely used in conversation.

For the interrogative form, simply reverse the verb "to be" (am, is, are), putting it at the beginning of the sentence:

Is she your sister? (She's your sister?)
Are we on the right road? (We're on the right road?)
Are you my classmate? (You're my classmate?)

Source: http://www.brasilescola.com/ingles/verbo-to-be.htm