For

for var = startvalue to lastvalue
  line of code
  line of code
end for

OR:

for var = startvalue to lastvalue step increment
  line of code
  line of code
end for

For allows you to repeat a block of lines for a given number of times.
The for line is evaluated the first time it is entcountered so the block of lines might not be run if the condition is not true.
Example: for i=2 to 1
The startvalue and lastvalue are computed once and for all at the begining of the loop. Those values may be expressions with variables. Even if those variables are altered within the loop, that will not change the behaviour of the loop which has been defined by the for line.
For uses your control variable var and increments it by one, at the end of the for loop. An optional level of control is possible: instead of incrementing by one, the step statement allows you to choose the increment. (See example 2)
Two other statements can be used in all loops:
- break; to exit from a loop (example 3)
- continue; to interrupt the current iteration and start over from the begining of the loop, after having carried out the incrementation. (example 4)
Loops of any kind can be nested, but not intertwined.

Correct:

for i=1 to 5
repeat
..
..
until a = "" $
end for

Incorrect:

for i=1 to 5
repeat
..
..
end for
until a = "" $

Example 1:
(Don't forget the space between end and for!)

// Table of 4
for i=1 to 4
print(int(i)$, " * 4=", i * 4, "\n")
end for

1 * 4=4.0000
2 * 4=8.0000
3 * 4=12.0000
4 * 4=16.0000

Example 2:

for j=3 to 1 step -1
print(j, "\n")
end for

3
2
1

Example 3:

for k=1 to 10
print(k, "\n")
if k = 3
break
end if
end for

1
2
3

Example 4:

for l=1 to 4
if l = 2
continue
end if
print(l, "\n")
end for

1
3
4

Notes:
The test is carried out from the first iteration, the incrementation (or decrementation) is done at the end of each iteration.