How to prompt participants in forums
Forums offer public and private prompting options to encourage further detail and discussion.
As well as being able to post prompts in the main reply box on the thread (either using the quote option or a new reply), you also can post private prompts to users. These prompts will only show to those with admin/observer rights over the topic and the original user, they will not show to the rest of the participants.
On their initial post, click on the discuss with user icon () and you will get a private reply box show for you to type your comment:
If you tick the 'Email this user to alert to reply' box before posting your reply, they will receive an email to notify them about your comment; you will also receive a notification when they reply to you should you use this option.
Private prompts are clearly identifiable in the discussion: