The three parameters condition, then and else are all expressions. The first one, condition, is interpreted logically. The two other ones, then and else, can be of any type. They should preferably be of the same type. If condition is true, the function returns the value of the expression then. If condition is false, the function returns the value of the expression else.
Use a simple if expression as mentioned by jagan in the expression box, if there is any syntax errors the window itself will highlight whether the expression entered is correct or not. If syntax is correct, it will show "Expression OK". You can see this on the top-left of the window.