Be:

Example:

You are rich (are=be verb).
To make it negative, we can say
-You are not rich/ You aren’t rich.

But we can’t say say
-You don’t are rich.

Similarly to make a question, we can say
-Are you rich?

But we can’t say, 
-Do you are rich?

So instead of adding an auxiliary ‘do’ with the verb ‘be’, we can invert the verb ‘be’ with the subject.