You can send anyone a private message if you don't feel like leaving a public comment. There are 2 ways to do so:
1. Click to that person's profile
. You can get there by clicking on their name or avatar in comments, discussions, the home page, members pages or the chat. Right under their name and avatar is a link called "Send a Message"
. Click on this link and you'll be brought to a page where you can write and send your message.
2. You can also go straight to your inbox and compose a new message from there. You'll find a link to your inbox at the right side of all pages.
Next, click on compose, found at the right.
From here, you can either send your message to an email address (type it into the box) or choose from one of your friends. You can't just type the name of someone, this box is only for actual email addresses. Plus, you can only choose from people who have confirmed you as a friend (whereas method #1 works for anyone).
Feel free to send me a message as practice!