Whilst editing a program, press PRGM and then press 4
Congratulations! you now have a For( in program!
This is the command for a For loop. The syntax is
For(VAR(A~theta),starting value, end value[, increment]
.code goes here
End
The part in brackets is optional. If you leave it off, the increment is 1.
What a For loop does is it sets VAR to the starting value, executes the code, adds Increment to VAR, then checks to see if VAR is > (< if increment is negative) the ending value, and loops if it isn't.

I think For loop syntax doesn't change much, if at all, between languages, it's a pretty common structure, but it can't hurt to check. Oh, and I forgot to mention that an increment of 0 is never valid

No, that's not it. In fact, a For loop sets the variable to the starting value at the very beginning.
What I meant was that
For(foo,bar,baz,0
is invalid code in any language, because the increment is 0