Sooner or later you are going to want to split a long line or join two short lines together. The F5 key will split a line at the current cursor position into two lines. The F6 key will join the cursor line and the following line together. In practice, you normally split a line on a space and no longer want the space after the split. The F5 key checks if the character under the cursor is a space and if so, deletes it before the split. On a join, F6 checks to see if the cursor line ends in a space and, if not, appends one before joining.
If you wish to keep a space on a split or suppress the append of a space on a join, then use a Ctrl–F5 or a Ctrl–F6. These keys make no assumptions; they simply split and join. Note that the operations of split and join are symmetrical. Try splitting and joining several lines to get used to this function.