Newbie needs help with User Defined Functions

Well I have never worked with SQL Server before and I am trying to figure things out on my own. I have references that I have been using to teach myself how to use SQL Server. I have run into a problem thou. I have to use an if statement in this User Defined Function. But I'm getting compiling errors on the IFs, ELSE IFs and the THENs. Can someone please tell me what is wrong with the syntax of my function. I have pasted it below. Any help given will be very much appreciated. Thanks in advance.

CREATE FUNCTION pbaweb.MonthTransForm
(
@.MonthNumber CHAR(2)
)
RETURNS CHAR

AS

BEGIN

IF(SELECT PayMonth from GroupActivity) = "1"

THEN

DECLARE @.MonthTitle VARChar(15)
Set @.MonthTitle = "January"
RETURN @.Monthtitle;

ELSE IF (SELECT PayMonth from GroupActivity) = "2" THEN
set @.MonthTitle = "Feburary"
RETURN @.Monthtitle;
ELSE IF (SELECT PayMonth from GroupActivity) = "3" THEN
set @.MonthTitle = "March"
RETURN @.Monthtitle;
ELSE IF (SELECT PayMonth from GroupActivity) = "4" THEN
set @.MonthTitle = "April"
RETURN @.Monthtitle;
ELSE IF (SELECT PayMonth from GroupActivity) = "5" THEN
set @.MonthTitle = "May"
RETURN @.Monthtitle;
END IF;
END;Hi,

Check BOL for IF ELSE syntax. THEN is not part of IF statement in SQL

IF Boolean_expression
{ sql_statement | statement_block }
[ ELSE
{ sql_statement | statement_block } ]

Best of luck|||DeepakKhattar is correct, there is no THEN, you would use BEGIN with END.

IF Boolean_expression BEGIN
{ sql_statement | statement_block }
END

However in your case you should look at the CASE statement
SELECT @.MonthTitle = CASE
WHEN PayMonth = "1" THEN "January"
WHEN PayMonth = "2" THEN "February"
WHEN PayMonth = "3" THEN "March"
.
.
.
ELSE "December"
END
FROM GroupActivity