can somebody help me to rewrite this? (3 replies)

Hi guys, i really need some help here. I plan to move my forum from the root domain to a subfolder named "/forum"

I need to rewrite from:
www.mywebsite.com/showthread.php?t=123456

To

www.mywebsite.com/forum/showthread.php?t=123456

I hope you guys can help me to find out the rewrite rules.
Thank you!