Treinta y uno & Treintauno?

Treinta y uno & Treintauno?
 

Hello,
i am beginner of Spanish language.

I have a question:
all numbers from: 16-Dieciseis... to... 29-Veintinueve are written in the same form.

Why is there a change from number 31? Does all numbers from 31 have to be wriiten in form: 31- Treinta y uno 73- Setenta y tres 99- Noventa y nueve

Is there any reason why this change?

Thanks

The shorter forms are the change, actually. The other numbers are not shortened.

Welcome to the forums, by the way!