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.
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.