DIM a
DIM b

WHILE a < 20

  a = a + 1

  IF a = 10 THEN
    b = b + 21
  END IF

  ?  "This is line no. ", a

WEND

?  "The value of b = ", b

Result:

This is line no.  1
This is line no.  2
This is line no.  3
This is line no.  4
This is line no.  5
This is line no.  6
This is line no.  7
This is line no.  8
This is line no.  9
This is line no.  10
This is line no.  11
This is line no.  12
This is line no.  13
This is line no.  14
This is line no.  15
This is line no.  16
This is line no.  17
This is line no.  18
This is line no.  19
This is line no.  20
The value of b =  21