upper$() -- convert a string to upper case

 
u$=upper$(a$) 

The upper$-function accepts a single string argument and converts it to all upper case.

line input "Please enter a sentence without the letter 'e': " l$ 
p=instr(upper$(l$),"E") 
if (p) then 
	l$=lower$(l$) 
	mid$(l$,p,1)="E" 
	print "Hey, you are wrong, see here!" 
	print l$ 
else 
	print "Thanks." 
endif 

This program asks for a sentence and marks the first (if any) occurence of the letter 'e' by coverting it to upper case (in contrast to the rest of the sentence, which is converted to lower case).