Hello Chris, and welcome to NeuroTalk.

In answer to your question... if you want to reply to a thread, you're doing it just right.

If you want to reply to some-one privately, then click onto their user name (on the left of your screen, and above any avatar they might have). You will then get a drop down menu.

The second option on the drop down list says 'send a private message to.....'

If you click that one, you can send whatever you want, to the person that you chose.

I hope that helps.