Domain question

Im about to buy a domain for my altervista wordpress site (9€45+tax)
So, I would like to know if the website will really use the domain, or just create a bad redirection;

Example if we would like to link at forum page:
I want => mywebsite.com/forum
I don't want => mywebsite.com
I hate => mywebsite.altervista.org/forum

I don't want to waste my money if the domain service will not be 100% compatible with my altervista site, that's why I ask before...
Thank you, Sincerely.

[karl94]

The domain woks mostly as a redirect, you can activate the cloaking (show the actual site inside a HTML frame) but the actual website will remain on the imnt.altervista.org domain.
Using the cloaking feature and a bit of JavaScript you can however always show on the address bar the second level domain URL.