Find a post by that person and click on the user's name. From the drop-down menu, choose "Send e-mail to..."
You can also use the site's message system.
From the drop-down menu, choose to view the profile. From that page, choose the "Contact" tab and one of your choices is to send or message. You can also send an e-mail from that screen.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.