Notes in Tumblr are not designed like a typical commenting system in that people can only comment on certain posts, and comments aren't replied to below the posts. The most common way to reply to a comment in your notes is to create a new text post and reply to the comment in the body of the post. While this isn't the most convenient method for replying to notes, it works because only people who follow you can comment on your posts, so your posts appear on their dashboard.
Click the "Notes" button in the upper right corner of your post beside the number that denotes how many people have replied, reblogged or liked your post. If you're on your dashboard, the notes appear below your post; if you're on your blog, this link takes you to your post page.
Select and copy the entirety of the comment, which is formatted as "username replied to your post: Username's comment." Open a new text post and paste this comment into it. This pastes the comment with links to the user's blog and your post, and shows the comment as a block quote. If you have the browser add-on Missing E, click the "Reply" icon on the far right corner of the user's comment to open it in a new text post automatically, though this only works on the dashboard and not your personal page.
Enter your reply to the user below his comment. Because this is a text post, you can add as long or short a comment as you want.
Tag the commenter's username in the "tags" section of your post. While this is not necessary, if the user has her own name as a tracked tag, it alerts her that there's a new post with her name attached.
Click "Create Post" to post your reply to your dashboard.
Tips & Warnings
- Users often have a conversation reminiscent of comment threads by reblogging a post back and forth, adding their own comment each time they reblog.
- Implement Disqus for a more standard commenting system. Some themes have built-in Disqus support, while others require you to add the system manually using the custom HTML feature (see Resources).
- The user's initial comment and your new post are public and viewable by all of your followers.