It's polite to explain why you are unable to accept an invitation or offer.
For example:
I'd love to but I'm busy, I'm visiting my sister this weekend. 

Everyday English with Teacher Alyse

Not Accepting an Invitation

From the Book Pocket Rainbow:
Chapter: Scarlet
In this chapter, Ntombi as various recollections of her mother -- some positive, some traumatic. We understand why she's on the street at age 14. 

Please note: there are thousands of colours in a rainbow, not just seven. The titles of the chapters reflect that. 

Pocket Rainbow

Scarlet

Second chapter of the story Pocket Rainbow. We are introduced to Joe Mbuti, a truck driver and his wife. 

Cyan

Have you ever been kicked out of home to live on the streets? Our protagonist finds herself on the street in a beggarly neighbourhood. She has to make her way in life - and knows only of one way of keeping alive: prostitution. What will she do? How will she navigate her new life and go to her beloved school?
The first story of the book Pocket Rainbow is called by the same name. The first chapter is Prism. 
Note" 'pap' is corn porridge