If you want to say something is not true + a form of to be:
just add 'not' after am/is/are.
I am not | I'm not | - |
You are not | You're not | You aren't |
He is not | He's not | He isn't |
We are not | We're not | We aren't |
They are not | They're not | They aren't |
Now do exercise 11 and 12 on page 35 in your workbook.