In the classroom

The verb "to be" in negative statements:
Das Verb "to be" in Verneinungen:

I am not silly.

He is not tired.

We are not lazy.

You insert the word "not" after the verb "to be". Here you can also use the short form:
Man fügt das Wort "not" ("nicht") hinter dem Verb "to be" ein. Hier kann man auch die Kurzform verwenden:

He isn't tired

We aren't lazy.

but (!): I'm not silly.