Vi Editor Question:

There are three basic modes of vi:

Command mode
This is the default when you enter vi. In command mode,
most letters, or short sequences of letters, that you type
will be interpreted as commands, without explicitly
pressing Enter . If you press Esc when you're in command
mode, your terminal will beep at you. This is a very good
way to tell when you're in command mode.

Insert mode
In insert mode, whatever you type is inserted in the file
at the cursor position. Type a (lowercase letter a, for
append) to enter insert mode from command mode; press Esc
to end insert mode, and return to command mode.

Line mode
Use line mode to enter line oriented commands. To enter
line mode from command mode, type a colon ( : ). Your
cursor moves to the bottom of the screen, by a colon
prompt. Type a line mode command, then press Enter. Any
sensible command from the Unix line editor ex will work,
and a few are good to know about. These commands are
indicated in this handout by a colon in front of the
command. Each time you use a line mode command, you must
type a colon to enter line mode, then type the command by
the colon prompt at the bottom of the screen, then press
Enter when you finish typing the command. (The search
commands starting with / and ? work similarly.