Setting up my first SQL Server 2005 Update/Insert Trigger

I am trying to create a trigger that will set the value of one field based on the values of two other fields. I first try to determine if I'm in an Insert or Update situation, then I make sure one of the two fields I'm interested in has actually changed, then I set calculate the third field's value.

I would appreciate feedback on whether I'm doing this the right way or not. I got most of the ideas from this forum. In addition, I'm currently getting errors "Incorrect syntax near 'TRY' in both locations.

My table structure (partial):
CREATE TABLE [dbo].[PairPalWatch](
      [WatchId] [int] IDENTITY(1,1) NOT NULL,
      [CheckFrequency] [smallint] NOT NULL DEFAULT (3),
      [DateLastChecked] [datetime] NULL,
      [DateNextCheck] [datetime] NULL)

And my trigger:
CREATE TRIGGER PairPalWatch_Update_trg
   ON  PairPalWatch
   AFTER UPDATE, INSERT
AS
BEGIN
      -- SET NOCOUNT ON added to prevent extra result sets from
      -- interfering with SELECT statements.
      SET NOCOUNT ON;

    -- Insert statements for trigger here
      declare @DateLastChecked datetime
      declare @CheckFrequency smallint
      declare @DateLastCheckedOld datetime
      declare @CheckFrequencyOld smallint
      declare @WatchId int

BEGIN TRY

      select  @WatchId = ins.WatchId,
                  @DateLastChecked = ins.DateLastChecked,
                  @CheckFrequency = ins.CheckFrequency
      from INSERTED ins

      IF EXISTS(SELECT WatchId FROM deleted)
            BEGIN
            select  @DateLastCheckedOld = del.DateLastChecked,
                        @CheckFrequencyOld = del.CheckFrequency
      from DELETED del

            IF ((@DateLastChecked <> @DateLastCheckedOld) or (@CheckFrequency <> @CheckFrequencyOld))
                  BEGIN
                  UPDATE PairPalWatch
                        SET DateNextCheck = CASE @CheckFrequency
                              WHEN 0 THEN DateAdd(dd , 1 , @DateLastChecked)
                              WHEN 1 THEN DateAdd(ww , 1 , @DateLastChecked)
                              WHEN 2 THEN DateAdd(ww , 2 , @DateLastChecked)
                              WHEN 3 THEN DateAdd(mm , 1 , @DateLastChecked)
                              ELSE DateAdd(mm , 1 , DateLastChecked) END
                        WHERE WatchId = @WatchId
                  END
            END
      ELSE
            BEGIN
            UPDATE PairPalWatch
                  SET DateNextCheck = CASE @CheckFrequency
                        WHEN 0 THEN DateAdd(dd , 1 , @DateLastChecked)
                        WHEN 1 THEN DateAdd(ww , 1 , @DateLastChecked)
                        WHEN 2 THEN DateAdd(ww , 2 , @DateLastChecked)
                        WHEN 3 THEN DateAdd(mm , 1 , @DateLastChecked)
                        ELSE DateAdd(mm , 1 , DateLastChecked) END
                  WHERE WatchId = @WatchId
            END
END TRY
BEGIN CATCH
    PRINT 'ERROR'
END CATCH  
END
GO

CheckFrequency translates to:
    0    Daily
    1    Weekly
    2    BiMonthly (every two weeks)
    3    Monthly

ADDED: When I comment out the BEGIN TRY, END TRY and CATCH lines, the Trigger works fine.