If

if expr
  line of code
  line of code
.
.
end if

Or:

if expr
  line of code
  line of code
else
  line of code (alternate)
  line of code (alternate)
end if

if allows your programme to execute one or several lines only if a condition is true.
With 'else' you can indicate some alternate lines to run when the condition is false.

Example 1:

a = 12
if a < 20
print("a is less than 20")
end if

a is less than 20

Example 2:

a = 12
if a < 20
print("a is less than 20")
else
print("a is greater than or equal to 20")
end if

a is less than 20

Example 3:

a = 21
if a < 20
print("a is less than 20")
else
print("a is greater than or equal to 20")
end if

a is greater than or equal to 20

Notes:

• There cannot be any sub called or code after the test as is the case in other languages. The same thing is true about else lines.
• When doing tests, if you want to do them in a string context, you have to put the dollar sign ($) right at the end of the test, and only once.
  Putting several dollar signs is incorrect:
  if a = b$ and c = d$
  This is correct:
  if a = b and c = d $
  If you want to combine tests in different contexts, you have to use brackets:
  if (a = b) and (c = d$)
  Note where the dollar is; that is inside the brackets for the second test to be done in a string context.