Instructors can make private replies to students in Forums. (Note: This capability is NOT enabled in Open Forums.) 

To make a private reply that only the student can see in a Forum:

1. Open the student's post and click on Reply.
2. Enter your message and click on the box labeled Reply privately.

Your private response will be seen solely by that student and only that student will have the post counted in their "unread post" tracking report. It will be completely invisible to other students. Note that the message is posted for students without a Reply button, so it is a one-way message from you and not the beginning of a two-way dialogue via the forum.

This illustration shows the instructor view of a private reply. The student view is the same except for the command links—all students can do is create a permalink to the reply (which only the student who received the private reply will be able to view) or view the parent post.