How do I define a function

You need to return a value in your function because you are saying that you are going to return a value by using the -> String part of the function. This code would compile

func greeting(person: String) -> String {
    println("Hello \(person)")
    return person

}


greeting("Tom")

Also, you'll need to add the \ that I added before person to get it to print out the variable. Also, if you don't want to return a value you can just delete the -> String part like this

func greeting(person: String){
    println("Hello \(person)")
}


greeting("Tom")

Hello Michael.

I have noticed 3 things that needs changing in you code:

1) you missed the "\" before "person" in the string interpolation

2) you forgot to surround Tom with "" when calling the function, it should be greeting("Tom")

3) you put "->String" after the function parameters which I do not think is needed

This would work fine:

func greeting(person: String) { 
  println("Hello \(person)") 
} 
greeting("Tom")