If

The if statement provides decision based branching.

if booleanExpression then
   scl statements
else   // optional
   scl statements
end if;

The boolean expression is what you would expect it to be. Here are a few examples

//make RD1 echo the inverse of RD0
wait on RD0;
if RD0 == '1' then
   RD1 <= '0';
else
   RD1 <= '1';
end if;

//test pin voltage
if RD0 > 2500 mv then
   RD1 <= '0';
   RD2 <= '0';
end if;

//test boolean var
if testBool then
   RD1 <= '0';
   RD2 <= '0';
end if;