mid$() -- return (or change) characters from within a string
The mid$-function requires three arguments: a string and two numbers, where the first number specifies a position within the string and the second one gives the number of characters to be returned; if you omit the second argument, the mid$-function returns all characters up to the end of the string.
Note, that you may assign to the mid$-function, i.e. mid$ may appear on the left hand side of an assignment. In this way it is possible to change a part of the variable used within the mid$-function. Note, that that way the length of the string cannot be changed, i.e. characters might be overwritten, but not added. For an example see below.
input "Please enter a string: " a$
for a=1 to len(a$)
if (instr("aeiou",lower$(mid$(a$,a,1)))) mid$(a$,a,1)="e"
print "When you turn everything to lower case and"
print "replace every vowel with 'e', your input reads:"
This example transforms the input string a bit, using the mid$-function to retrieve a character from within the string as well as to change it.