Like all social media, blogging isn't just shouting into the void of the Internet -- it's a conversation. Whether you have the answer to somebody's question or want to add your two cents to a discussion, you can respond to a post in several ways. One of them, reblogging, is always available, while the others require a specific set of circumstances. If you reblog, your response will display both on your own blog and on the blog post to which you responded; if you use the other methods, it'll display only on the blog post to which you responded.
Click the "Reblog" button on the post. You can do this both from your dashboard and from that individual post's page on its blog, but only if you are signed in to your Tumblr account. Depending on the theme used by the blog, there may also be a "Reblog" button or link on the blog's main page.
Type your response to the post directly beneath the quoted text. By default, all reblogs are done from your primary Tumblr blog -- if you want to reblog from a side blog, switch to it using the drop-down menu on the posting form.
Add tags, if you want, and then click "Reblog Post" to publish your response.
Click the "Reply" button. This button looks like a speech bubble and displays only on your dashboard. Moreover, it shows up only on posts from blogs for which the owner has enabled replies; for you to be able to reply, the owner of the blog must follow you, or you must have been following that blog for longer than two weeks.
Type your response in the pop-up window. Replies are limited to 250 characters, so keep the response short.
Click "Reply" to post your response. It will display on that blog post's individual page, in the same location as likes and reblogs, and will be identified as coming from your primary blog.
Click inside the text field at the bottom of the post. This text field displays only if the owner enabled answers while creating the post; you can find it on your dashboard, on the post's individual page and on that blog's main page.
Type your response into the text field. Answers are limited to 140 characters. If you are signed in to your Tumblr account, the answer will be identified as coming from your primary blog; if you are not, you will need to leave a name and email address.
Click "Answer" to post your response. It will display on that blog post's individual page, in the same location as likes and reblogs.