You will find the following examples of conditional statements in the Revit help menu if you search for 'conditional'. I wanted to let you know what I learned from these...
First of all, the conditional statement goes into the Formula field of the parameter that you want to control. This formula will automatically fill out the Height value of the example below. Formulas are like a language and Syntax is the spelling.
Now to translate...
The example below is an either/or situation. It's saying 'If the Width is LESS THAN 3000mm THEN the Height will be 2000mm IF NOT (LESS THAN 3000mm) THEN it's 1000mm.
The commas are acting as a THEN and a IF NOT THEN.
This can be taken further by having multiple variables. The example below is found in the help files and provides the syntax for such a case..