When using Tumblr's default rich text editor, adding a "Read More" break to your post is easy -- insert a blank line, click the Plus icon that appears, and then click the gray bar with three dots. At first glance, however, there is no way to add a break using the HTML editor, and switching back and forth between editors every time you want to add a break is not only cumbersome but, if done at the wrong time, will remove any inline CSS styling you might have added. Adding a break using HTML without switching back to the rich text editor is, instead, done by adding a short piece of code.
Inserting the Break
Type [[MORE]] where you want the "Read More" break to appear; it must be between the closing tag of the preceding paragraph and the opening tag of the next.
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This is the same code that appears if you add the break using the rich text editor and then switch to the HTML editor; you will see that [[MORE]] has appeared in the HTML code. You're simply skipping a step and saving yourself some time by adding it in this manner.
The [[MORE]] break code is not standard HTML code and won't display correctly in the editor's Preview mode, appearing instead as plain text. It will turn into a true break after the post is saved, whether you publish it immediately, queue it, schedule it or save it as a draft.