1. Мне не 10 лет, мне 8.
2. Я не Катя, я Маша.
3. Ему не 9 лет, ему 7.

I'm not ten, I'm eight.
I'm not Kate, I'm Mary.
He isn't nine, he's seven

I am -> I'm
You are -> You're
He is -> He's
She is -> She's
It is -> It's
We are -> We're
You are -> You're
The are -> They're

I am not -> I'm not (только один вариант. Строго не I amn't)
You are not -> You're not / You aren't
He is not -> He's not / He isn't
She is not -> She's not / She isn't
It is not -> It's not / It isn't
We are not -> We're not / We aren't
You are not -> You're not / You aren't
They are not -> They're not / They aren't