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The If..then..else statement

The If .. then .. else.. prototype syntax is

_________________________________________________________________________________________________________ If then statements
-- if statement if- expression -then- statement------------------------- -else- statement -|
___________________________________________________________________

The expression between the if and then keywords must have a boolean return type. If the expression evaluates to True then the statement following then is executed.

If the expression evaluates to False, then the statement following else is executed, if it is present.

Be aware of the fact that the boolean expression will be short-cut evaluated. (Meaning that the evaluation will be stopped at the point where the outcome is known with certainty) Also, before the else keyword, no semicolon (;) is allowed, but all statements can be compound statements. In nested If.. then .. else constructs, some ambiguity may araise as to which else statement pairs with which if statement. The rule is that the else keyword matches the first if keyword not already matched by an else keyword. For example: 

 If exp1 Then
   If exp2 then
     Stat1
 else
   stat2;
Despite it’s appearance, the statement is syntactically equivalent to 
 If exp1 Then
    begin
    If exp2 then
       Stat1
    else
       stat2
    end;
and not to 
 { NOT EQUIVALENT }
 If exp1 Then
    begin
    If exp2 then
       Stat1
    end
 else
    stat2
If it is this latter construct is needed, the begin and end keywords must be present. When in doubt, it is better to add them.

The following is a valid statement: 

 If Today in [Monday..Friday] then
   WriteLn ('Must work harder')
 else
   WriteLn ('Take a day off.');

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