EDT can be set to automatically wrap lines (end the current line and advance to the next line) when you approach the right-hand margin while typing.
Use the line-mode command SET WRAP n, where n is the desired right-hand margin. Most of the time, 75 is a good value for a typical 80-column display.
This feature works on one line at a time and will not justify the remainder of a paragraph when you insert text. However, you can select a block of text to fill (see "Formatting Text").
To disable this feature, use the line mode command SET NOWRAP.