What are the differences between the following commands?

a. i and I
b. a and A
c. o and O
d. r and R
e. u and U

a. The i command puts vim in Insert mode, enabling you to insert characters

before the current character. The I command moves the cursor to the begin-
ning of the current line before putting vim in Insert mode, enabling you to
insert characters at the beginning of the current line.
b. The a command puts vim in Insert mode, enabling you to insert characters
after the current character (this command appends characters to the current
character). The A command moves the cursor to the end of the current line
before putting vim in Insert mode, enabling you to append characters to the
end of the current line.
c. The o command opens a line below the current line and puts vim in Insert
mode. The O command opens a line above the current line and puts vim in
Insert mode.
d. The r command replaces the current character with the next character you
enter; it is not necessary to press ESCAPE after using this command. The R
command replaces characters until you press ESCAPE.
e. The u command undoes the last command you gave. The U command
undoes all the changes you made to the current line since you most recently
moved the cursor onto the line.