simple yes?
 

Okay, this may be simple, may not, I'm not too good at PHP/MySQL Though if I have some code to work with I can usually figure out what I need.

Lets suppose I have a page called test.php theres nothing on it but a text box and submit button.
Also it is located in the root directory of the website will the forum is located in /forum/
How would I make it so its a check to see if the username exists in the forum? ex: I enter spandexman and press submit, it checks and if it is then it says okay, if not then it says not in database or something?

Simple, yes. What is your ultimate goal here? Checking upon registration?

Yes, I want it so they can submit parts in different parts of my website, but I want them to enter the correct username and password in the form that is not attached to the forums for verification to prevent spam etc.