s:if

Executes a block of code if a certain condition is met.

INPUT
<s:if true="3 {{=}} 3">
  Yes. {{ = }}
</s:if>
OUTPUT
Yes.
true=""  

Run block of code if the condition is #true.

INPUT
<s:set var="my website name">Awesome</set>
<s:if true="{{my website name}} {{ = }} Awesome">
    My website is awesome!
</s:if>
OUTPUT
My website is awesome!
false=""  

The opposite of true=. It executes a block of code only if a certain condition is not met (is equel to #false).

INPUT
<s:set var="x">4</s:set>
<s:if false="{{x}} {{ = }} 3">
   4 is not equal to 3!
</s:if>
OUTPUT
4 is not equal to 3!
return=""  

Return #true or #false instead of executing the content of if.

INPUT
<s:if true="1 {{ = }} 1" return>
   Hi there!
</s:if>
OUTPUT
#true

Usually we omit the content when we use return=, like this:

INPUT
<s:if true="1 {{ = }} 1" return />
OUTPUT
#true